Now, lemme explain this for all y'all: In Winter Soldier, a video shows Peggy talking about how Cap saved the man that would eventually be her husband, but never says his name. Same with the Agent Carter TV show. Then, in Endgame, The Ancient One says that when you take a stone from a timeline it makes a new one. But only when you take one. So, the theory, that the screen writers publically said was their theory/cannon, is that Cap was always Peggy's wife, and that they just kept the fact that he was alive, a secret. So, in this timeline, there were 2 Caps, 1 in ice, and 1 growing old with Peggy. Now, whether or not the old man is Steve is up for debate, but I'd like to believe that there were 2 Steves in Earth 616, and the old Cap lived a secret life with Peggy. And, before you say that he wouldn't sit around and let bad stuff go down, 1) Those things might not have happened in Earth 616, 2) Steve knew that everything in history would lead to Thanos' defeat, and that if he changed anything, not only would he be found out, and the Cap-cicle wouldn't be found, but Thanos might win, and 3) Cap was a soldier, and a some point, every soldier's career comes to an end.