SPOILERS about Avengers: Endgame ahead.
I agree with Jonah Goldberg more than I disagree with him, but sometimes a guy has to stand firm in what he believes. Of all the complaints one can make about the last of the Avengers movies (at least in this cycle), inconsistency about its treatment of time travel is not one of them.
I should note that I think the movies’ conception of how time works is pure fantasy not applicable to reality, but if one accepts their physics framework, the story is just fine.
Seemingly in order to make a fun reference to the Back to the Future movies, the smarter characters explain that it isn’t possible to go back in the past in order to change the present. The heroes live in a world where the bad guy, Thanos, has used the Infinity Stones to wipe out half of all life in the universe. The problem is that the people whom they left behind when they went into the past would still be in a future in which Thanos had already accomplished his goal. It would be along a different time stream that Thanos had failed. Undoing what has already been done is a logical impossibility if we accept a tangible universe.
Jonah’s complaint is about the end of the movie. Once the world is saved, Captain America travels into the past to return the stones to their proper times, and he doesn’t return. It turns out he’d decided to stay in the past and live out a lifelong retirement with his one-time love. But then… there he is, as an old man sitting nearby a few seconds after the younger him had gone into the past. Weren’t we already told that changing things in the past put you on a different time stream?
Yes, but we’ve also seen evidence that two copies of the same person could exist alongside each other. Indeed, Captain America had to fight with himself! Jonah’s complaint is that Captain America’s staying in the past would have changed reality in all sorts of unpredictable ways, but as long as he stayed quiet and lived as a regular Joe far away from the action of the Avengers, he would have done nothing logically incompatible with the world of the story that we’ve been following over the past eleven years. For all we know, he was out there all along.