JoBlo.com’s The Good, The Bad, and the Badass: Robert Downey Jr.
"By design, the Marvel universe on film is bigger than any one actor. I truly believe that as good as they are, Chris Hemsworth or Chris Evans could bow out of their respective franchises and you could still get more CAPTAIN AMERICA or THOR movies. In fact, a change seems inevitable at some point. But I truly think Downey Jr is the only guy that could ever play Tony Stark. He’s the anchor for the entire Universe, bringing a cheeky charisma to the films that’s tough to replicate (although Chris Pratt has a lot of potential on that score) and I couldn’t imagine an AVENGERS movie without him.
Arguably, he made the Marvel films what they are today, with him injecting so much of his own personality into Stark that the movies played to a wider-audience than they would have otherwise. He brought in a whole new group on moviegoers (specifically non comic-book fans) that ended up sticking with the various franchises well beyond THE AVENGERS to the point that the Marvel audience is arguably the broadest of any franchise going today, which is why all the films are so huge. Kids, teens, young adults, parents, these days everyone seems into Marvel and that simply would never have happened without Downey Jr’s big-time charisma and tireless promotion of even the Marvel films he’s not involved in.”
[read more: career retrospective]