The first three phases of the Marvel Cinematic Universe, known collectively as The Infinity Saga, are a masterclass in retroactive continuity. Take a tesseract, some Aether liquid, and a Samuel L. Jackson cameo that happened after the movie was over, and suddenly *poof* three movies about superheroes has turned into a story about Infinity Stones. Moreover, according to Endgame writer Christopher Markus, this Avengers: Endgame theory about Old Steve Rogers retconning the character into the Infinity Saga should be treated as correct.
To recap, Avengers: Endgame ended on a weird timeline twist that left many fans uncertain if they'd understood the time travel scenario correctly. Steve Rogers took extra vials of Pym particles when he and Tony were in the 1970s after seeing Peggy Carter. He was then sent back in time to put the various Infinity Stones back at the end of that trip, he used those extra particles to go one stop more, back to the 1950s. There he stayed, marrying Peggy Carter and living out his life with her.
When Bucky Barnes and Sam Wilson turned from the platform where Steve was supposed to return to, instead they found a very old Steve Rogers on a park bench. He'd come there to wait for them, to explain what he'd done and pass the Shield to Wilson.
How could this even make sense? For Rogers to go back in time and marry Peggy contradicted everything in Phase 2 and 3. That includes an interview with Peggy in the Russo Brother's own Captain America: Winter Soldier, where she claimed to have gotten married and had kids. But note her wording, at 2:20: "He saved over a thousand men, including the man who would become my husband." No details, only that it was Rogers who engineered it.
Not to mention, there was also Peggy Carter's funeral in Captain America: Civil War. Rogers, the young version, was a pallbearer. There is no sign of Old Steve. Or is there? According to one theory, he's been there the whole time.
Check out her funeral, specifically at the 14-second mark, the man who is diametrically opposite on the casket from Rogers.
This might seem like a far-fetched fan theory, but according to the Endgame writer Christopher Markus, he buys it, 100%. Speaking at San Diego Comic-Con, he said:
I would like to believe... that there are indeed two Captain Americas in the MCU timeline. That Steve Rogers who looped back into time has therefore always been there, and that he is living somewhere else in the movies you're watching. And what I really like to believe is that there's an old man at Peggy's funeral in Civil War, and that's old Steve Rogers... Can I explain it scientifically? Not really, no.
Another clue: Peggy was very confused when young Steve turned up in her hospital room. Rogers believed she was losing touch with reality. But what if her confusion stemmed from knowing there was an older version visiting her at the same hospital?
Perhaps this twist is the biggest retcon of them all. But it's one worth believing in.