The only time the time travel makes any sort of sense is in the first movie. In T1, it is demonstrated that time travel results in a closed loop, and no changes to the timeline can be made. Any attempt to change is actually going to fulfill the timeline. That is, a T-800 was always sent back to kill Sarah Connor, Reese was always sent back to protect, Reese was always the father of John. The fun thing is that no characters, human or otherwise, are aware of this. Only the audience knows, since only the audience sees the Polaroid photo in both the future and the present.
T2 doesn't really contradict the rules of T1 when you remember that absolutely none of the characters actually know that time travel is a closed loop that cannot be changed. Thus, both Skynet and Sarah/John/Uncle Bob/Dyson try to change their fates without knowing that it's not actually possible. Even after the good guys win, the movie ends with Sarah narrating that the future is a dark road at night, and they don't really know anything.
Now, I did fib a bit, there is a contradiction to the T1 time loop in T2--the death of Miles Dyson. Surely if Uncle Bob had detailed files on Dyson, he might have mentioned to Sarah that Dyson died before the launch of Skynet (assuming a closed loop). So Dyson's death would be a change to the timeline. However, this can be explained if you imagine that "detailed files" contain, say, a middle name and address, which is much more than Skynet had on Sarah Connor in the first film. If Skynet didn't have files on the circumstances of Dyson's death, then the time loop theory can still hold through T2 (which sucks for the characters after all their efforts to prevent Judgment Day).
Every subsequent movie totally shits on the rules of T1 and, pretty much, every other Terminator before it. I remember a reviewer commenting that the rules have changed since T3 Arnie says "Judgment Day is inevitable". As if he would fucking know that. Then again, under the rules of T1, he'd be right. Sequels from T3 onwards break the previous rules both from the characters' and audience perspective. T3 demonstrates a changed timeline and surprises John by saying it's not a prevention but a delay. Salvation also has a changed timeline, none of what John knows is really relevant anymore, and Skynet somehow knows everything about John and Kyle, like wtf, that doesn't work if everything's changed, right? Genesys has a fucked up series of changed timelines, changed movies, mashups between movies, and a time travel forward via a time machine made out of like, dishwashers and microwaves or some shit.
Now Dark Fate has decided that the future can be changed, drastically, but the same shit will still happen and, what's more, we can now receive terminators traveling back from multiple possible futures. (I know Kyle referenced "one possible future" in T1, but he doesn't actually know he's in a closed loop, so of course he's open to possibilities.) Apparently, they decided to draw the line here at the point of time travel departure. So, if you get zapped back to the past, then anything that would change the future to prevent you getting sent back won't actually prevent you getting sent back. So all of Skynet's terminators can still arrive in a changed timeline in which Skynet never exists. I
t makes some sense, I suppose, as long as you choose where to draw the line of what changes can affect. Avengers Endgame puts them in an identical parallel alternate universes' past, so they couldn't change their own timeline at all. Back to the Future puts a grace period on changes, they slowly fade in, giving you a chance to fix things if you want. T1 is a closed loop (as a twist, since nobody knows until the end, and then only the audience knows, not the characters). Dark Fate allows changes, but anyone sent back is never prevented from being sent back. Sucks for John.
I will say that Carl and Sarah were kind of fun, but otherwise, I was disappointed that it was another T2 rehash, essentially. I mean, the T1 rules have been long since broken, so I can't really care about that anymore with sequels. I don't think this franchise is salvageable at this point.