But in the end, I don't actually think that's a bad thing. There is a lot of variations you can do, even within such a limiting structure. Summer blockbusters are the pop music of movies. And just like pop songs, even if you know the overall structure you can still be surprised and entertained throughout.
And, just like music, it doesn't mean that there can't be things that break the mold entirely, even if they aren't quite as popular. There's always going to be someone out there pushing the boundaries, and there's always going to be someone really skilled who makes something really popular that doesn't conform to the formula at all.