Who exactly determines that?
As far as the studios are concerned, box office returns is all that really matters. If a movie is considered brilliant by critics and fans alike but still does crap at the box office, it won't be considered any less of a "flop" than movies that sucked and did poorly, such as Green Lantern.
The only way for there to be "over-satuation" is for there to be more movies coming out greatly exceeds the rate at which people go to the movies in the first place. Which for me is about once a week. So, feel free to let me know when they're putting out 53+ "blockbuster" superhero films a year and I'll consider conceding your point. Though in all likelihood I'd probably just increase the rate at which I go to the theater if it's a hero I'm interested in.