Originally Posted by
Wermys
Long Spoilers discussing how the movie works
There is a lot of confusion on how the movie works. One, it is entirely consistent. Rule #1, time travel is impossible to go into YOUR past. Rule #2 Time Travel is NOT impossible to go into alternate reality past. #3 Every action taken in the alternate reality past creates a new timeline that continuously branches. Those are the rules in this movie.
For those people confused how Captain went into the past married Peggy and became an old man without ruining the timeline. Well, the fact is, he has always gone into the past in this timeline. That has always taken place, and will always take place. The old man is Steve Rogers, it is a Steve Rogers who came into this timeline some time in the past. And now exists in the current. That Steve Rogers could very well be the same Steve Rogers, or a different Steve Rogers. Frankly it doesn't matter at all. The point is that he is Steve Rogers. Now as far him going into the past. Remember. When he went back to the second time to return everything. He is going to the same timeline/alternate universe that he got them from because of Tony's device. Now him depositing the Stones/Mace actually makes a third Timeline. The Second timeline continued until the point in time where he returned the Stones, or didn't return the Stones. When he at the end where instead of returning to reality he stayed with Peggy, at that moment it created another time/alternate universe. Now when Falcon and Bucky in the future see old Steve. That old Steve is not the same Steve that they knew necessarily. It is a Steve who came back into time and jumped into this universe. The key to this working is if the whole Universe is from Steve's point of view. Since from his perspective he went back in time to do this. As for Bucky and Falcon, from there point of view he has also always gone into the past and come forward. Nothing is changes at all. Since time is always moving forward for the current POV, never ever backwards from Steve's POV. All that happens is that it just keeps creating branchs. That is it. As the ancient one states. This is not there reality. This is a constant that happens in this universe is that you can't travel to your OWN past. But you can hop into the time stream and create a new branch with a change.
Things to consider in the future. This movie firmly attached one rule in place. You can't go back and change your past. It also established that it is possible to go to alternate versions of reality/time/dimension. This means that you could see someone else pop into this timeline from another timeline. So it is entirely conceivable that Loki for example hitched a ride with Thanos, or that in the timeline/reality where they all went Thanos disappeared and is no longer heard form again, while the stones still exist in that universe thanks to Steve bringing them back. Anyone this is how I interpret this all working. Essentially, this is a comic universe and these things are extremely common in comics and they are bringing this as an obvious possible feature to the MCU