No matter how much money Avengers: Endgame makes at the box office or how high its Rotten Tomatoes score sits, there will always be people who complain about it. In this case, it's not just a matter of people tearing strips off something that lots of people love (though there are, inevitably, lots of websites and individual writers being edgelords about how "over-rated" the film is because they're big babies), it's because the film DOES have story issues caused by its own complexity.
To a certain extent, the Russos should have stuck with their own party line and said "f*ck what you expect from time travel movies because of what you've seen before, this is different," and we would have had to accept it. The old "well, because he's Batman" line of conversation, we'll call it. But they couldn't let it lie, because they are great people.
In a round-table discussion of the movie with EW, the Russos have addressed some of the big questions hanging over Endgame and one of the most interesting and simultaneously confusing is the question of Captain America's ending.
In the film, Steve Rogers heads through time with the Infinity Stones in his knapsack and Mjolnir on his arm to return the Stones and restore the timeline and cut off some branches the Avengers created. But he also ignores his duty to preserve the main timeline by staying in the past and marrying Peggy Carter (as well as giving her the dance of a lifetime) and just completely messing up her life as it had been. It's a nice mess-up, but it's still not something we can ignore.
And then to add more confusion, Cap still manages to get to the same point in time in the future as he left, to hand the Shield over to Sam Wilson and anoint him the new Captain America. Which makes no sense, because he can't have changed events in the past and arrived at the same point - he would have been splintered off into another reality. So what gives?
Well, according to the Russos, he has been living in another dimension for the past seven decades and just pops back to do the hand-over. As Joe says:
“If Cap were to go back into the past and live there, he would create a branched reality. The question then becomes, how is he back in this reality to give the shield away?”
Yes, that is the question, because Captain America cannot simply hop around through dimensions. And nor can the technology he has available to him. He CAN just about get away with jumping through time, because despite it contradicting the need for a Quantum Engine, you can sort of suggest that his Quantum suit has a system built in to allow subsequent jumps into the Quantum Realm (even though it is a MASSIVE contradiction of the film's own rules) because we see it before then when Tony and Cap jump back from 2012 to 1970. But the Quantum Suits aren't established as inter-dimensional.
What's missing here is an explanation that the Quantum Realm isn't just the key to time travel but also to jumping into alternate realities at will too. A crossroads of sorts that govern both time and space, but across every single one of the branches created by every infinitely tiny decision. Think about how many realities there are out there and your brain will hurt very quickly and yet Cap seemingly managed to jump from the one he created by marrying Peggy back into the "main timeline."
That's a story worth telling, and it seems like the Russos are hinting that it might be told:
“Interesting question, right? Maybe there’s a story there. There’s a lot of layers built into this movie and we spent three years thinking through it, so it’s fun to talk about it and hopefully fill in holes for people so they understand what we’re thinking.”
In a Chinese Q & A, the Russos also doubled down on that when they were asked what time Cap goes back to meet Peggy. (The answer is obviosuly the 1940s, not just because the cars there are pristine 1940s models but also because it makes NO sense for him to go back and marry anyone but "his" Peggy):
Peggy Carter was probably already married and in her mid 40s in 1970, in that case what year was it that Captain America went back to dance with her? A: We can't answer it for now, this is a story that happened in an alternate reality. Maybe it will be revealed in the future.
Could this be their launchpad for reintroducing Cap further down the line in another reality? Could we see him jumping through other versions of the MCU timeline? Could he even look different (to get around Chris Evans wanting to leave the role)? The possibilities are endless when you're this vague.
And even more interestingly, if this suggestion of such causal inter-dimensional travel is indeed true, could this be the key to introducing the concept of mutants and the Fantastic Four to the MCU? If so, that's huge!