Yes and no. There are two established methods for mucking with time in this storyline. One is to travel through the Quantum Realm, which as established in “Ant Man” (and arguably “Dr Strange”) is not so much a realm as a conduit connecting all versions of reality. A mortal thrown into this realm on his own is likely to be lost forever, neither here nor there for all eternity (as nearly happens to Michelle Pfeiffer’s character in “Ant Man and the Wasp”). But with the aid of the handy doo-dads in “Endgame,” it’s possible to pinpoint specific moments in time and space and “hop out” of the Quantum Realm where – and when – you want to. So if 2019 Avengers living in the reality we’ve become used to in these films (let’s call it Earth Prime, as long as we’re stealing DC’s IP) use the Quantum Realm to travel back to Earth Prime’s 2012, then as soon as they show up there, they’ve created a new timeline, just by virtue of their presence.
That part we have “explained” to us several times. But there’s still a way to change the past without necessarily creating a new timeline, and that is to use the godlike powers of the Infinity Gauntlet. With combined mastery over reality and time (and several other things), the Gauntlet stands apart from any rules of physics, reality or logic, and if it’s possible for it to make true a wish to “return everyone who disappeared 5 years ago and while you’re at it, destroy Thanos and all his troops”, then it should be equally possible for it to make true the wish, “erase the last five years of history and return us to 2018 except with Thanos dead.”
This would have been nice, because as pointed out elsewhere on the web, the “bring everyone back 5 years later” thing creates as many problems as it solves. People would return to find their loved ones have died in the intervening 5 years due to illness, injury or old age, their homes now occupied by new owners, their jobs filled by someone else, their fortunes spent by executors or heirs, etc. The thousands of people who vanished while on a commercial airline flight will reappear at 30,000 feet with no plane around them, folks driving down the highway when they vanished will reappear moving 60mph with no car, etc. The courts would be flooded with disputes over property and finances for decades to come: if you owned and operated a successful corporation, vanished, then reappeared to find someone else in charge – someone legally and properly voted into the job by your Board of Directors – who is the rightful claimant to the job?
The film pokes fun at “Back to the Future’s” version of time travel – that you can change your own timeline – and really in a way, it’s right. When Marty changes the circumstances of his parents’ courtship and thus the nature of their relationship, he effectively creates a new timeline. A timeline in which George McFly is confident and successful and so are his children. A timeline that includes a version of Marty McFly, but it can’t be the SAME version. When Marty “returns” to 1985, it’s a different 1985, one that logically would’ve been inhabited by a Marty McFly with a whole different life experience and mindset. In addition to that spiffy new truck he finds in the garage, he should also find a doppleganger Marty living in his bedroom!
Endgame takes a more logical approach, sort of, but they make a hash of it. When Steve goes back in time and stays there, what exactly happens? If he lives out the next few decades in a separate timeline, then it must be one where Peggy also knew, loved and missed him, or else how can there be a reunion? But for that to be true, then there must be another version of Cap stuck in the ice in that reality, and so “our” Steve is stealing that Steve’s Peggy! He’s the only person in that reality who knows “Captain America” is not dead, but rather stuck in the ice somewhere, and he keeps that information to himself so he can give himself a happy ending. That’s not very “heroic,” is it? Alternately, if he somehow manages to live in the “Prime” timeline…creating two Steves in one universe…and we simply haven’t seen him until now, then there are other problems. If he’s been married to Peggy in “our” reality, it would’ve had to be a secret marriage, so secret he didn’t even come out of the shadows to appear at Peggy’s funeral for Pete’s sake. Some husband. And for a lot of that history, he’d still have had the powers and abilities of Captain America, but kept himself secret so as not to upset his “happy ending.” So the champion of America watched quietly and did nothing as history unfolded tragedies he knew were coming: JFK, MLK, the Oklahoma City bombing, 9/11, etc. Nice.
But he has to be in another timeline, or else where does he get the shield he hands to Sam Wilson at the end? The shield from our timeline is destroyed, and we know its whereabouts until that happens. So not only did he steal the happy ending owed to another universe’s Cap, he also trudged up to the Arctic and stole the shield out of his frozen hands!
Actually, I’ve mentally written my own solution to that one: Steve goes back to 1945 with the aim of rescuing his alternate self so that guy has the chance at a happy ending. But on finding the plane, he learns the other Steve is not frozen, but in fact dead (so it’s already a new timeline). He buries that Steve, takes the shield and vows to live enough life for the two of them.
Anyway, Endgame is a mess, but it’s a fun mess (which makes it a perfect capstone to the MCU). I’m grateful to get any ending at all to 10 years worth of storylines, and now I can jump off and be done with the whole thing, not the usual way I leave a franchise – in either boredom or disgust – but with an actual ending. And thanks to Amazon’s “The Boys,” I can throw the last shovel full of dirt on the whole superhero genre.
