*SPOILER*** the time travel issue
So my understanding is that the way time travel works in this film is based on multiverses. So essentially, when they travel, they’re not going back in time in their reality, rather they’re travelling to a different reality. All their actions in that reality simply affects that reality while the one they travel from remains the same. This is the reason given for why the snap couldn’t be prevented by time travel (i.e. preventing the snap in another reality does not change their own) and also the reason why the ancient one is so protective of her own reality.
I think this was a great approach to time travel that was completely undone by cap’s ending.
So he travels in time to return the stones to their realities and chooses not to come back. This is fine and I’ll assume in that reality, the Steve Rogers there is still frozen in ice so there technically wouldn’t be two Rogers running around. But the problem is that back in the reality he comes from, Steve is still present and has become an old man. This is impossible based on the film’s own time travel rule. Steve travelled to a different reality and found his life there. This does not affect the reality he comes from. In the reality he comes from, he is simply gone. The realities do not intersect.