Basically, the problem with BTTF, Terminator, etc is that they use time travel as a way to change the present/future. Except there's a glaring and fundamental flaw with that plot point because it ignores one's own relationship with time. For example,
the Grandfather paradox: if you went back in time and killed your own grandfather you would essentially prevent your own birth thus you'd prevent yourself from going back in time to kill your grandfather thus your grandfather would still alive be therefore making your birth possible therefore making it possible for you to go back in time to kill your grandfather.
BTTF sort of flirts with this idea with the subplot of Marty having to make sure his parents get together to ensure his birth. More to Banner and Stark's point, BTTF completely ignores Marty's relationship with time (the logic is that the DeLorean enables him to do so). He does go back 30 years into the past,
but time is still moving forward for him. Now this is where it gets tricky and it's what Banner was trying to explain to Scott during the movie. The basic hypothesis is that
to time travel to the past you must still move forward which is a contradiction and raises all sorts of problems and questions. Endgame itself tries to address this by giving us the scene of baby Antman and old man Antman, essentially a form of time travel. We actually see the sort of problems and questions the hypothesis causes at the end of the first BTTF when Marty goes back to the present only to find it changed. Why does the Marty at the end of the film turn out like the exact same person as the main Marty despite the new 1985? Why does Marty retain his memories of the old 1985? Does this mean there are two 1985's or is there only one that was changed?
(Sidebar: If you've seen the film Kimi no Na wa/Your Name you might already have an idea of what I'm talking about. Frankly, it's the only film I've seen that's done "changing the past" logically.)
So what does this all mean? It means that going back in time to prevent Thanos from using the Infinity Stones doesn't change the fact that it
already happened. This is what they mean by "time travel to the future is possible; time travel to the past is not" and this is why Banner and Stark weren't so gung ho about changing Thanos's actions so much as
undoing it. That's what separates Endgame from BTTF and Terminator. Endgame wasn't about trying to change the past like they were, and to a certain extent you can argue that's why it's okay for Cap to stay in the past with Margaret.