How’s Old Steve In The Original Timeline?
And here’s the big mystery that comes from Cap going a-time-traveling. To the surprise of his friends, Steve doesn’t return when expected but they soon discover an old man sitting by the lake. This is Old Man Steve, who reveals he decided to stay in the past and live the life that was denied to him the first time when he fell in the ice. It gives him a happy ending and brings his story full circle…but it does clash with the previously-established rules of time travel.
If Cap went back in time and married Peggy, this surely creates a new branch of the timeline. Going by how Hulk described things at the beginning of the film, this would mean that Cap would grow old in a fresh version of the future. So, from the heroes’ point of view, he should disappear and never return.
The only way this would work if going back in time was always Steve’s destiny and he hasn’t changed time by doing it (eurgh, my head hurts). However, this is not the case, as Peggy discussed her husband in the original timeline – a fellow soldier Cap encountered during the war – in The Winter Soldier.
It could be argued that maybe Steve used the time-GPS to navigate back from his timeline to the original universe so that he could be there by the lake at this point of Avengers: Endgame. That doesn’t feel like the intention, though, and the filmmakers likely just went for emotion here rather than plot logic.