The theory isn’t completely out of the realm of possibility. After all, when Steve comes back to the prime timeline as an old man, Bucky reacts in a manner that suggests he already knows what lies ahead. However, it looks like the theory is only partially true. When asked by The Hollywood Reporter if Steve Rogers first offered the mantle of Captain America to Bucky Barnes, Avengers: Endgame writers Christopher Markus and Stephen McFeely had this to say:
So, it’s settled then. Sam Wilson will be the next Captain America. Well deserved if you ask me. Avengers: Endgame is currently playing in Malaysian cinemas. (Source: The Hollywood Reporter)
