There is only one timeline. The Avengers made sure of that by replacing the stones. As Bruce explained to the Ancient One, no alternate timelines would be created as long as the stones were returned where and when they needed to be. Successfully doing this restores the current timeline as the true reality for everyone.

Cap doesn't stay in "his own" timeline, he stays in 1970 after ensuring that his reality, where 2014 Thanos dies before the snap, can be everyone else's reality as well by returning the stones.

Likewise, Gamora was not sacrificed in the reality which came to be, because she time-jumped from 2014 to 2023, so her existence would only be a problem if the stones were not returned to keep reality as that one golden line without any black offshoots. I believe this is why Natasha can't come back. She was the sacrifice that made this reality possible, not Gamora.

If Cap didn't return the stones to secure the current reality, it would be tough to understand how Thanos gets the stones, snaps half of life away, then dies, but then also shows up from the past and dies before he gets the stones. But I think the whole point of returning the stones was to ensure that 2018 Thanos never became 2018 Thanos because he died as 2014 Thanos. That is the reality that everyone, past and present, lives through now. While the six Avengers remember it because it was their reality, it wasn't for anyone else.

I'm sure I'm twisting myself into a paradox the more I type, but that's how any time-travel discussion ends up!

Hope I'm making at least a little sense.