Now that Iron Man 3 has made pretty much all the money in the world, there's reportedly a contract dispute brewing between star Robert Downey Jr. and Marvel, over whether he'll be back for Avengers 2 and 3, and possibly Iron Man 4 as well. Is Downey Jr. overpaid? We don't know. But we do know he's nearly irreplaceable.
There has been a ton written about the Downey Jr./Marvel contract fight the past couple of days. Short summary: because Downey Jr. has a good agent, he got a percentage of Avengers' phenomenal income, and he stands to make a psychotic amount of money for Iron Man 3. But he wants more, to keep playing Tony Stark. Meanwhile, some of the other Avengers stars are also asking for more money, and they reportedly see Downey Jr. as their "big brother," sticking up for them. Read more about this here and here.
How much money should Robert Downey Jr. get? This is a question on which none of us is an expert, unless you're Robert Downey Jr.'s accountant or you work for Marvel. The past couple of days, people who aren't either of those things have been holding forth about whether Downey Jr. is demanding too much money, and it seems like there's some kind of "baseball player" thing going on, of wanting to hate the greedy talent. But unless they have to charge $100 for tickets to see Avengers 2, Robert Downey Jr.'s compensation does not affect you directly.
But meanwhile, there is a question about which all of us can have an equally informed opinion: Can Robert Downey Jr. be replaced? And the answer is, probably not. They can recast Tony Stark with a cheaper actor, but he probably won't be as popular a character afterwards.
There are a couple of reasons for this, which boil down to: 1) Nobody except for comic-book fans had heard of Tony Stark before Downey Jr. took the role, and the popularity of the character is entirely tied to Downey Jr.'s performance. There just wasn't that much love of Iron Man before he came along, and these days he even looks like Downey Jr. in the comics. 2) Tony Stark is kind of a douche, and it's a rare actor who can play that sort of role without becoming annoying or loathsome.
Sure, they recast Bruce Banner and James Rhodes already, but neither of those roles was associated with an actor who had nailed it the way Downey Jr. has with Tony Stark. (Sorry, Edward Norton fans.) Watching Iron Man 3 really cemented how much Downey Jr. has owned this role, and how much of a complex, sympathetic-but-obnoxious character he's turned Tony Stark into. Any actor who takes on that role immediately after Downey Jr. is going to be compared to the original — and probably won't come off that great in the comparison.
And yes, they will have to recast Tony Stark eventually, because Robert Downey Jr. is not immortal. Eventually, they will have to make Iron Man like James Bond or Batman — a character who gets recast over and over again, with a new actor in the role. And when that day comes, if they're smart, they will have a whole new take on the character. Just as Roger Moore's Bond is barely the same person as Sean Connery's or Daniel Craig's, or Christian Bale's Batman is hardly the same guy as Michael Keaton's. A post-RDJ Iron Man will have to be, if not a full reboot at least a soft reboot.
Again, there's nothing intelligent that any of us can say about how much Robert Downey Jr. deserves to be paid — and maybe, there's no way to make the math work and Marvel really has to cut him loose. But Avengers sequels without Downey Jr. probably won't do as well, and a hypothetical Iron Man 4 would be way less interesting as well.
Replacing Downey Jr. as Tony Stark, while he's still able to play the role, risks creating an underwhelming result at best, and an epic disaster at worst. If they can't get Downey Jr. back, then the best thing they can do is to give the character a long rest. (Or just have him show up in some action sequences, in the suit, but never show him out of it, I guess.)