Let's open this post with a few stipulations.
STIPULATION ONE: The Marvel Logo is the biggest box office draw out there. Bigger than any star, bigger than any character. You can slap the Marvel Logo on a movie starring a psychotic raccoon and a grunting tree and it will gross more than $300 million domestic and almost top the yearly box office chart.
STIPULATION TWO: That being said, it seems pretty clear that within the Marvel Cinematic Universe Robert Downey Jr.'s Tony Stark is the biggest box office draw going. The worldwide grosses for the Iron Man flicks are, by far, the highest in the MCU:
Compare Iron Man's average worldwide gross to Thor:
And Captain America:
And The Incredible Hulk, which grossed just $263 million worldwide.
STIPULATION THREE: Marvel already seems to understand that Robert Downey Jr.'s Tony Stark is the most important single portion of the MCU. This is why, after The Avengers grossed $1.5 billion worldwide, it was announced that he received a $50 million payday for his appearance.
OK, so, with all these things stipulated, here's my question:
Why doesn't Marvel lock RDJ down forever?
I'm imagining something like Magic Johnson's "lifetime contract" with the Lakers, but exponentially bigger. One. BILLION. Dollars! Obviously, there'd need to be some rules worked into the deal. Say, lay it out so he only has to "star" in one film a year (an Iron Man film, an Avengers film) and "guest star" in one or two more. Maybe have him show up on an episode or two of Agents of SHIELD. That would allow him plenty of time to pursue smaller projects such as The Judge, if he wishes.
Marvel already seems to be considering such a workload, if early reports about the latest Spider-Man reboot are to be believed: supposedly, a large portion of that film will involve Spidey going up against Iron Man to determine if he's worthy of joining The Avengers. With renewed cooperation between Sony and Marvel, that gives Iron Man a much larger universe to play in. Imagine if Marvel could work out a similar deal with 20th Century Fox, which holds the rights to Fantastic Four and X-Men. Exciting times!
Why lock him into a long term deal? Well, for starters, continuity: this is a promise to the fans that we'll have at least some of the same faces throughout the Avengers films, that by the time we get to the Infinity War flicks it's not Black Panther and Ms. Marvel doing battle against Thanos. It would send the same signal to Fox: "Let's work together, because we're in this for the long haul!"
The biggest variable, of course, would be Robert Downey Jr. himself. Why would he want to lock himself in? What does he have to gain? Well, for starters, he gets to be the highest paid actor in history—a record that would likely stand for all time. We live in a post-star age. It wouldn't really make much sense to offer such a deal to anyone else.
Plus: Who doesn't want a billion dollars to do what they love doing?