I've never really thought about it that way. I had to do some Wikipedia-ing to get an idea of just how much material they have to work with. Marvel alone has 20 big name superheroes. They also have another 949 "superheroes" listed at the bottom, but they include some really minor characters. Out of the major ones, I think there has been a movie or television show about all of them in the last ten years. Well, the Fantastic Four and Silver Surfer were more like fifteen years ago. Maybe they'll actually end up running out of material soon. I wonder if DC still has anything up their sleeves.