Honestly, I think that's selective memory at work. I would say most superhero films of the last ten years have ranged from mediocre to outright terrible. But in discussions like this one, people tend to remember Dark Knight and Iron Man and forget all about The Spirit, Wolverine, Punisher, the Fantastic Four movies, etc.
Or rather, subjective appraisal. You're almost certainly right that the bad ones sink while the good ones remain vibrant, but even at that, an honest accounting of (my own) likes and dislikes in the modern era of superhero films finds it to be pretty top heavy.
Awesome: X-Men, Spider-Man, Batman Begins, Spider-Man 2, Iron Man, Dark Knight, Watchmen
Good: X-Men 2, Daredevil (Director's Cut), The Incredible Hulk, League of Extraordinary Gentlemen, Hellboy, V for Vendetta
OK: Fantastic Four, Spider-Man 3, Ghost Rider, Wolverine
Meh: Punisher, X-Men 3, Hellboy 2
Teh Suckz: Fantastic Four 2, Elektra, Hulk, Superman Returns
Haven't seen: Punisher: War Zone, Catwoman, The Spirit
Granted, those last films are likely to gets classified in the lower tier if I ever watch them, but even at that, the genre's doing well where I'm concerned. And for the sheer pleasure of the films in the 'awesome' category, I'm more than willing to put up with the occasional stinkers like FF2 or Superman.
Fictitiously yours, Trent Roman