what happens when dc and star wars stomp on that at the the box office?
With what? The last three Star Wars flicks made a ton of money but not Avengers money. DC has nothing that will come remotely close to Avengers money. DKR was around 1billion and DK was about the same but Avengers was 1.5billion.
They better lock down Downey with some options to reprise the role a few more times if they want to move the series forward. It is too soon to recast Iron Man/Tony Stark. Hulk is kind of interchangeable since Hulk is the main there, you always see Stark even with Iron Man.