Let's say you were Steve and you had the chance to stay with the girl you loved and live a quiet happy life with her, or you could run around in the past fixing all the wrongs. If you chose the latter, what would be your plan? You mentioned WTC attacks. If you could travel to any point in time and had knowledge of the future, how would you stop that event?
I'm asking because I don't think it'd be that easy. Butterfly effect will change things in unexpected ways, and eventually those changes will make the new timeline stray away so far from the original that your future knowledge becomes pretty much useless. Like for example, if Cap had decides to save Bucky from becoming Winter Soldier, that would mean Tony's parents never got assassinated by Bucky. But how would Tony's parents being alive affect the power struggles of Stark Industries, and how likely would it even be for the events to align so that Tony becomes Iron Man again in the new timeline? And if there's no Iron Man, what will that mean when cosmic threats would later threaten the Earth?