Mickey Mouse is protected by trademark, which doesn't expire as long as the owner maintains it. The stories of the cartoon shorts that Mickey starred in will, barring an act of Congress, fall into public domain eventually, but Mickey, the character, will not. Mickey will never be seen (legally) outside of Disney property. I can't say for sure whether Marvel superhero characters are protected by trademark, but I think it is safe to assume they are. It is advantageous to do so for recurring characters like cartoon stars (e.g. Mickey, Popeye, Daffy Duck) or superheroes, so I can't imagine why they wouldn't be. Long story short, Disney is not going to be able to wait for the Marvel characters to fall into public domain because that simply will not happen.