With that said, hear me out, I think I just broke the Russo Brothers time travel theory and proved that Markus & McFeely theory is the only one that works within the rules set in Endgame:
If the Russo Bros. theory is correct and Steve WASN'T living his life with Peggy in his own timeline, the moment he would supposedly "go back from the future" to his own timeline in the past - the only way he could travel back and not show up in the platform was if he was traveling back from a future point - and deliver a shield that wasn't even on our own timeline, he was ALSO making a branch on our MCU timeline.
So what I'm saying is simple, really: there are only two possible explanations for Cap NOT to showing up at the platform and Sam and Bucky seeing him on that bench instead:
a) Cap went back from a point in the future, which means that the moment he does this, let alone Steve gives a new shield to Sam, he creates an alternate timeline, according with the Russos Bros. own rules;
b) He was in the MCU timeline all along. That explanation works even if he does goes back, because it means that simply going back in the timeline doesn't create an alternative timeline.
So which one is it? Are Markus and McFeely correct or the Russo Bros. when it comes to Cap going back to Peggy?
My theory - that aligns with Markus' and McFeely's explanation, is quite simple:
You can't kill baby Thanos because what happended in the past, can't be altered. But that doesn't mean that Cap wasn't supposed to go back to the Peggy of his timeline all along, hence, time paradox.
Back to the Future rules DO NOT apply here. Steve and Peggy are a very specific case, it's the single time paradox that happens in the film. Everything else indeed are branched realities. The only way to DOOM it is without the infinity stones, but the paradox is that Steve always was supposed to go back. So it isn't like killing baby Thanos, because that wasn't supposed to happen. Steve going back and staying on his own timeline means that that was supposed to happen all along. Hence, time paradox, not really Back to the Future rules. Steve's "future" after Endgame was always in the past.
There are 14 million futures in IW, and they only win in one. What happens if they don't win? The end of the universe, only to get replaced by a new one, Thanos says so. So if Tony doesn't sacrifice himself, the time loop never happens, because the universe ceases to exist. But Iron Man does win, and Cap completes the time loop going back to 1948 and living in secret with the Peggy of his own timeline.
So the screenwriters theory theory is actually accurate with the film, while Russos explanation makes it impossible for Cap's mission to return the stones to ever be accomplished, since by simply traveling back in time - according with the Russo Bros. explanation - an alternate timeline is created.
It isn't changing the past because Cap going back to Peggy was always how that would go down. Cap's not altering the past, he is living his present, which is in the past just after Agent Carter's show and the fallout with Souza, like the screenwriters explicitly explained.
