8 "Unanswered Questions" From Avengers: Endgame, Explained

8. Why Is Old Steve Rogers In The Current Timeline As Opposed To An Alternate Timeline? Why Is His Changed Past The Only Thing Affecting The Present?

He's not. As Bruce explains early in the film, anything the Avengers change in the past would create a split timeline. Director Joe Russo has made this clear about Steve's choice: "The time travel in this movie created an alternate reality. He lived a completely different life in that world."

Steve would have had a load of Pym particles with him to ensure he could put all of the stones back in one go, accounting for potential mishaps and bonus trips along the way. What he does in the past cannot change the present as both he and the audience knows it, thus creating an alternate timeline with Peggy. He would have to return to his old timeline following Peggy's death to be with Sam and Bucky in time for the exact moment they would notice him on the bench. There is no Steve Rogers living a secret life in the canon MCU timeline while he's simultaneously frozen in ice. He exists simultaneously in a different one instead.

While the rules of timeline jumping aren't explicitly stated as being possible with Pym particles the same way that plain time jumping is, this is the only way an old Steve Rogers could end up on that bench as he couldn't possibly have lived that life in his original timeline.