As Chris Evans mentioned, Robert Downey Jr. didn’t just play Iron Man (a.k.a. Tony Stark), he was the Marvel character. Just like Evan’s Captain America, it’s probably safe to assume many Marvel fans couldn’t handle someone else as Iron Man (not even Evans). This is especially true after the emotional and dramatic death of Tony Stark in Endgame. Unlike other superhero franchises and universes, Marvel rarely ever recast roles. At this point, too people see Tony Stark and Downey as one and the same for Evans to take over, but at least he was realistic about Downey’s presence within the Marvel universe.