caleblyn said:
What about Loki in NYC taking the Tesseract and poofing away? I guess we need to assume that Cap goes back to NYC and lets Hulk take the elevator.
That's a different timeline.
It's confusing they way they explain it in the movie, but you cannot change your own past. It happened. as soon as you travel back to your past, you instantly create a diverging timeline that is completely separate from your own. So in the MCU, time paradoxes do not exist, you just keep splitting off new timelines. So there is now an alternate universe where Loki escapes with the Tesseract. Sucks for those folks, but it does not effect the Prime timeline in any way.
When cap goes back to the 1940s to be with Peggy, he creates a new timeline where they get married and grow old together. That technically is not his Peggy, she still got married to someone else in the Prime timeline b/c you cannot change your own past. But with all intents and purposes, she is "Peggy-enough" b/c her shared history with Steve would still be the same up to that point that he traveled back to the past.
Funny observation on Reddit i saw: Imagine later in life when Peggy's Niece Sharon Carter is coming of age. Imagine creepy uncle Steve fondly looking at her b/c they hooked up in Civil War in the main timeline.