OK. I loved this movie. I loved, loved, loved this movie. It was like they basically made a movie specifically for me. There’s just one thing bugging me, but it’s bugging me a lot and it’s making me enjoy the movie in retrospect less than I want to. Posting it here in the hopes that someone can offer an explanation that makes me feel better about it 
Let’s talk about Captain America’s ending. Midway through the movie, when Banner is talking to the Ancient One, she outlines the rules of time travel in this fiction pretty neatly. You make a change to the past - like taking an infinity stone out of the timeline - and it causes a split in the timeline, with unpredictable results. This is why it was important to return each of the stones to its point of origin when the heist was done. OK, fine. Straightforward, simple, unfussy, and mostly the movie adhered to this.
So let’s designate the MCU timeline we’ve all been watching for fifteen years and call it Timeline Prime. The Avengers return to various points in Timeline Prime and do things that might result in a split to the timeline: take out infinity stones, have interactions with key characters, even drag Thanos five some odd years into the future. But by returning the infinity stones to their points of origins, presumably we are to understand that the Avengers obviated these splits. Thus Timeline Prime remains intact, if a bit twisty, and there are no splits.
But.
At the end of the movie, Cap decides to travel back to 1970 (or whatever year), where I guess he reunites with and marries Peggy Carter and gets his Happily Ever After. Now, according to the rules as I understood them to this point, this should have split the timeline. And if Cap had simply disappeared and never reappeared, Sam Beckett-style, it all would have made sense. Presumably he had his adventures with Peggy or whatever, but that’s a different timeline and this is not that story. I would have loved that ending, and you could still have him leave a note and the shield for Sam and gotten the same torch pass with a nice voice over.
But that’s not what happened! Instead, Cap appears right here in Timeline Prime as an old man, presumably having lived through all of the events of Timeline Prime as a second Steve Rogers. In order for this to be possible, this would have to mean that Transported Cap took no action that would cause the timeline to split. So he arrived in 1970 (or whenever) knowing that of his three best friends in the world, one will die alone on a planet light years from her home, one will die a young man leaving a wife and daughter behind and bereft, and one is being slowly brainwashed in a foreign land. You’re expecting me to believe that the Steve Rogers we have come to know over all of these movies wouldn’t have been on the first plane to Russia to try to rescue Bucky? That he’d just have done nothing about all the Hydra agents hanging around his wife’s workplace? That he’d not have intervened in a thousand ways, any one of which would have ensured that he’d never again be a part of Timeline Prime? That idea is such a betrayal of the character of Captain America as it’s been shown that I can’t accept it - and yet I can’t find any other explanation for him showing up in Timeline Prime at the end (with Nat and Tony still dead).
Somebody talk me down.