The reason why there weren't a ton of hero movies before 2000 is the technology wasn't around, and thus they weren't proven box office commodities.
Raimi's Spider-Man is a mega-hit and changes the game forever. Now we get four to six per year.
We can't really get more than that, because they cost so damn much. And while maybe one or two will underachieve, more often than not they'll make money. The huge expensive tanks in the genre aren't frequent.
And even if we get to a point where superhero movies bat .500 at the box office financially speaking, the hits are big enough that most studios will take the risk. Unless a colossal behemoth budget movie totally bombs and sinks a studio for good, we won't see studios back off the superhero trend.
I actually think it'd be riskier for studios to NOT try to make new superhero movies, because franchise fatigue is far more real than genre fatigue. If they just churn out sequels, the profits will diminish. They have to roll the dice and bust out the new heroes in hopes of finding that next gold mine.