WARNING: Spoilers for Avengers: Endgame ahead.
Captain America's Avengers: Endgame ending is not a plot hole. In the MCU culmination, the Avengers use time travel to pluck the Infinity Stones (destroyed in the present) out of previous Marvel movies so they can undo the effects of Thanos' snap. That leads to at least one new timeline by Loki, a bizarre time loop where Thanos disappears in 2014, and a lot of confused fans.
But what's caused most debate isn't the mission itself but the final use of time travel at the end. Because removing the Infinity Stones from the time stream can have catastrophic consequences, Steve Rogers goes back and replaces them all (as well as Mjolnir) at the points the Avengers took them. But instead of returning to the present, he travels to the 1940s to live the life he never got with Peggy Carter; Avengers: Endgame ends with an older Steve passing the Captain America mantle to Falcon before showing Steve and Peggy's fateful dance years earlier.
Immediately, some have cried plot hole. There are two perceived issues, both of which misunderstand the time travel methods used in Avengers: Endgame. Here, we'll explain how Captain America's ending really works and doesn't damage the timeline in any way.
To be fair to those confused, Avengers: Endgame's time travel is so confusing even the writers and directors don't agree. Going by the rules explicitly stated in the text, time is (mostly) linear - you can go back into the past and move around, but that always happened. The only way to actively change things is to remove an Infinity Stone as they define the laws of reality, and the movie makes clear Steve fixed all of those issues.
False Plot Hole #1: If Steve Ended Up With Peggy, Then The Captain America Movies Didn't Happen
The first false Avengers: Endgame plot hole is that for Steve Rogers to live his life with Peggy deletes his time as Captain America; that this happy ending is an alternative to his time in the ice and subsequent defrost. However, that makes a big mistake: it's talking about the wrong Steve Rogers.
The Cap who gets the dance with Peggy is the one from the year 2023 who's defeated Thanos, only now giving himself some closure in the past time where he was always frozen. There were two Steves during the 20th Century: the one frozen in the Antarctic and his ten-year-older self who'd traveled back in time to live with Peggy.
False Plot Hole #2: Steve Rewrote Peggy Carter & The MCU's Past
The second false Avengers: Endgame plot hole is that Steve Rogers essentially rewrote Peggy Carter's life, replacing her husband and children immediately and, quite possibly, having a knock-on impact to the development of S.H.I.E.L.D. and, by extension, the Avengers themselves. This takes everything just discussed about the time loop, then assumes that any action in the past will alter the MCU timeline, which isn't what's happening here.
The MCU has been purposely quiet about who Peggy Carter's husband is, only revealing that he fought in World War II and was supposedly rescued by Captain America. The truth now, however, seems to be that the husband was always Steve Rogers, perhaps living under a pseudonym to keep his existence a secret.
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Many aspects of Avengers: Endgame's time travel don't make total sense, but Captain America's happy-sad farewell isn't one of them. His dance is a fitting - and logical - end to a film and an era.
