Our original analysis of the first two films observed that, simplified, we had an uncaused cause, what is called a predestination paradox.  Simplified, Skynet comes into existence because it sends a terminator back to eliminate John Connor, who only comes into existence because the terminator came after his mother and only destroyed Skynet because of information obtained from time travelers from the future.  Thus Skynet tries to eliminate John because Skynet tries to eliminate John, and John is born because John is born.

The solution to the birth of John is that Sarah Connor had a child by someone other than Kyle Reese who became the problem for Skynet, so Skynet sent a terminator back which failed to kill her before the child was born, but did kill her later, so Sarah's child sends Kyle Reese back to protect Sarah, and Kyle replaces the original father, giving us John Connor.

The other problem, though, requires that there was an original version of Skynet that was not created by Cyberdyne, and that when Cyberdyne found the fragments of the T-800 smashed in its machines it was able to leap ahead of whoever was working on that original Skynet to produce a hardware-based system that was functional years before the original system would have been.

That meant, though, that when the Connors destroyed Cyberdyne history reverted to the original version of Skynet, and had a complicated anomaly as that original version faced John Connor and sent back a terminator to kill his mother, establishing the altered history and allowing time to continue.

T3 gave us this, as the new version of Skynet was a software version launched by a division of the Air Force, and as we observed would have been the original Skynet that Cyberdyne displaced.  We thus have preserved a history in which Skynet does exist, Terminator Salvation is possible, and time is preserved.

Our new movie removes T3, and decides that when the Connors destroyed Cyberdyne, they destroyed Skynet--only to have something else called Legion arise in its place.

This should be obvious at this point, but the confirmed destruction of Skynet destroys the entire history to this point.

Let's create the timelines briefly.

• Sarah Connor, waitress in California, gets pregnant in some liason about which we know nothing.  Her child grows up.  Meanwhile, someone somehow launches something that is given the name Skynet, which takes over our military systems and attacks us.  Sarah Connor's child organizes humanity to strike back, and is doing well enough that Skynet decides to eliminate the mother in order to eliminate the child, and having developed the ability to send living tissue back in time and created a killing machine with a living tissue exterior, it sends this first primitive model of a terminator back to 1984 to kill the waitress.
• Somehow Sarah Connor escapes and survives this killing machine long enough to give birth to her child.  At some point after that the machine kills her.  Since this does not prevent the birth of the child, time resolves to an N-jump.  Sarah Connor's child recognizes that the reason Sarah died was that a terminator was sent from that future time, probably 2029, to 1984.  The child, now a leader of the resistance, chooses to send someone named Kyle Reese back to protect Sarah.
• Kyle Reese arrives in a past in which a low-grade terminator is trying to kill Sarah, and together they escape the machine and cause it to be destroyed in the mechanisms of a Cyberdyne factory.  Kyle interferes with Sarah's liason and becomes father of her child, who is now probably John Connor, but he is killed in the fight.  Cyberdyne gets hold of the components from the destroyed terminator, and begins work on something that replaces the original Skynet with a more potent version.  The war in the future is tougher, and Kyle Reese is better trained.  The terminator sent back is more advanced.
• In a sense history keeps repeating itself, except that each time the terminator is more advanced and Reese is better trained.  Eventually they plateau, and since Skynet has failed to prevent the birth of Sarah Connor's child, who at this point is definitely John, history resolves and continues, the more powerful Skynet fighting against humanity.
• Still trying to defeat John Connor, Skynet on some unknown date after 2029 sends a T-1000 back to kill John.  John escapes, but the machine kills Sarah.  Because John survived, history resolves to an N-jump and John sends a reprogrammed T-800 Model 101 back ostensibly to protect himself but ultimately to save his mother.
• Having both escaped the T-1000 and saved his mother, John now works with his mother to learn how Cyberdyne created Skynet.  They destroy Cyberdyne and prevent the creation of that version of Skynet.

You can see where this takes us.  If there is no Skynet, there is no terminator sent back to 1984 to kill Sarah Connor, and the entire story unravels

In what is almost a throwaway scene we are told that John Connor is dead.  The essence of it is that when Skynet sent back a terminator it sent back several presumably to different points in spacetime.  One of those managed to find the young John Connor with his mother on a beach somewhere and kill him.

For what it's worth, this is more than just a dumb plot device to remove John from the action.  It is used as a motivator for both Sarah and Carl, and perhaps an explanation for why there is someone else leading the future resistance.  It's still an issue.

The problem is the infinity loop:  if in the future Skynet has trouble with John Connor, and sends terminators to the past to kill him (or his mother before he is born), and even one of them succeeds, then in the future Skynet will never have had trouble with John Connor and will have no knowledge or reason that would cause it to send any terminators to the past.  As with the destruction of Skynet, the death of John Connor unravels the story, unravels time, and starts everything over again in a perpetually repeating complex loop.

The erasure of Skynet and the death of John--and really, either of these events, but certainly both together--means that a lot of what this movie assumes simply can't be true.

We are told that Skynet never came into existence, and John Connor was never the leader of a resistance movement.  We've already noted that this means Skynet would never have sent anything back to kill anyone, and further the resistance would never have sent back any protectors.

Of course, we have that stretch from 1984 to 2029 and probably a few years beyond to the unknown date from which the T-1000 was sent, and indeed if during those last years Skynet decided to litter the timeline with terminators programmed to find and kill John Connor, doing it all in one shot (so that the success or failure of each does not influence the decision to send the others), they would be there, right up to the moment Skynet fails to send them, at which point we revert to the earliest moment in time when they do not arrive, and recreate that history in which they are not there.  It should also be noted that based on the departure date given by Grace, Legion is sending its terminators back after the latest departure date we can imagine for Skynet to have done so.  That means that in the history in which Legion comes into existence, Skynet never existed and never sent anything to the past.  Thus even were we able to move time past that moment (that is, ignoring the infinity loops here), we can't have anything in this timeline that is based on the actions of the now non-existent Skynet.

That obviously means that the terminator known as Carl never arrived in the past.  It never killed John Connor (although we should note that this would have been Sarah Connor's child, and not Kyle Reese' son).  It never discovered what it was like to be human, never figured out how to detect temporal arrivals, never sent coordinates to Sarah, and was not there to help when needed.

Perhaps less obviously, though, we do not have the version of Sarah Connor shown in the film.  That Sarah Connor remembers being pursued by a terminator, destroying Skynet, being John's mother, seeing him killed, and fighting terminators thereafter.  None of that ever happened.  She cannot have memories of a history she erased, because she never lived it.  She got pregnant in 1984 from a liason with someone she met one night, and birthed and raised a child either as a single mother or partnered with the father, and was never targeted by any machine from the future, and never met a time traveler.  She was a waitress, not a commando.

Obviously there are a lot of problems that arise because of the fact that the film doesn't fit the history created by the previous films, even when we eliminate all but the first two.  The question, though, is would it stand alone if we ignore those problems?

Here's the story of the film, stripped bare.

Somewhere in Mexico there is a girl named Danielle, or Dani to her friends.

At some point someone creates an artificial intelligence called Legion that takes over the military systems and begins a war against humans.  It attempts to use the strategy of turning humans against each other, but Dani arises and unites humans against the machines.  In the process she saves many people, including a young girl named Grace.  To help fight against the machines, some humans, including Grace, become augmented, people who are part machine so they can stand a chance against the machines.

Legion realizes that Dani is its biggest problem, and so it develops a time travel projection device (a machine that can send someone or something to a different point in time) and uses it to send one of its terminator machines, a Rev-9, back to perhaps 2020 to kill Dani before she becomes a problem, and before she has an army to protect her.

Dani survives, and then in the future after the terminator has been sent to kill her she sends Grace back to protect her.  Grace dies protecting Dani, but succeeds in destroying the Rev-9.

Here are our problems:

• If the Rev-9 succeeds in killing Dani, no one in the future will have any reason to send Grace back to protect her, because she was never a problem for Legion.  More complicated, because she was never a problem for Legion, Legion will not have sent back the Rev-9 to kill her, and we have an infinity loop.  Therefore, Dani has to survive.
• However, if Dani survives unaided, there is no reason for her future self to send Grace back to defend her.

In the original movie, the reason for trying to kill Sarah was always that it would result in the death of John.  We recognized that Sarah had to escape long enough for John to be born, but that once John was alive the terminator that had been instructed to kill Sarah could succeed in its mission, killing Sarah, but fail in the objective, killing John.  Thus John was alive and had a motivation to protect his mother, and sent Kyle back to do that.

The problem in this movie is that Dani is always the direct objective.  Despite Sarah's sexist thoughts otherwise, the plan was never to kill Dani so that her child would not be born, but to kill Dani herself.  That makes this more like Terminator 2, in which the objective was to kill John.  Our resolution of the situation in that movie was that John escaped, but that the T-1000 killed his mother, and so he sent a T-800 back ostensibly to protect himself but actually to save Sarah.

By the time this movie has gotten started, Dani has lost all her family.  Her father was killed before she knew she was in danger, and her brother was killed as they were fleeing their first encounter with the Rev-9.

That might be the way to resolve it.  If in the first altered timeline the Rev-9 killed Dani's father and brother, and Dani thought that sending Grace back might save her brother, that could get us the second altered timeline.  Grace did not succeed in saving Dani's brother, but Dani would not have known that that was why she sent Grace, and so it is likely that she would have sent her again.

All of this is ignoring one of the major problems with that first altered timeline.  How does Dani survive?  In the history we see, the Rev-9 was potent enough that it took the combined efforts of the trained and battle-hardened Sarah, the augmented human Grace, and the T-800 Carl to destroy it.  In our reconstruction, there is no experienced Sarah, no Carl from the erased history, and no Grace from the future.

To make this possible, we need to make two extrapolations.

The first is one we made for Terminator 3:  the first time Legion sends back a killing machine, it's not a Rev-9.  Maybe it's a Rev-3.  Whatever it is, it is not as potent nor as durable as the one we see, and by some means Dani manages to destroy it.  Then after Dani sends back Grace, Legion upgrades to the more powerful machine in order to deal with Grace.

The second extrapolation takes a cue from the film but changes it.  Somehow someone is hunting terminators.  It obviously is not Carl and Sarah, but someone has figured out how to detect travelers from the future and come to understand that they are not friendly, and has been developing weapons and tactics to oppose them.  Thus when the Rev-3 or whatever it was arrived, so did our present-day terminator fighter, the replacement for Sarah Connor.  He or she gets Dani through that first altered history, and then is there in the later histories when Grace is also there.  It is likely that this unknown assistant is a team, strong enough combined to replace both Sarah and Carl.

It appears that Terminator:  Dark Fate can work as a time travel movie, sort of.  That is, the core story works, but it fails in the details.  In order to remove Skynet the film had to remove everything Skynet ever did in the past, which includes sending T-800s and other terminators back to kill the Connors and turning Sarah Connor into a warrior.  Obviously a good part of the intention in making the movie was to include its two established character stars, but their presence makes the existence of Skynet essential, and the removal of Skynet makes them impossible.

If we assume that Sarah and Carl aren't really who the movie claims them to be but a pair (or perhaps team) of terminator fighters who have been detecting and fighting time travelers, we resolve those issues.  Ultimately we manage to create a time travel story that works, although it is touch and go--that is, as with the previous Terminator films, the heroes are not merely good, they're very lucky, and the outcome was unlikely but not impossible.

So stripped of all the baggage from the original Terminator films this would work, but burdened as it is with its attempt simultaneously to include that history and erase it, it fails.  It is unfortunate, really, because the film fails entirely because of its failure to resolve the temporal problems of the first two films, which was something T3 did quite successfully.