I think nu metal is dying.
It happens to all styles of music that are "over-copyed". It happened to just about every style of music scince the 80's. It happened to the Hair Bands, Grunge, and now Nu Metal. Even if a style of music doen't dye off, it is usually changed and evolves into a completely different style than it was when it started off, like punk and country.
I dont however think its dying off because of the lack of guitar solos or the length of the song, its just a fad. Like you said, U2 are a legendary band and they hardly have any solos, but they're original and have never been copyed. I think the genre of nu metal is just getting old fast.
And one other thing, I hardly think of Sum 41 or Blink 182 as nu metal. They're more like a rock/punk combination. Limp Bizkit, sure, but dont forget, they dont have a guitar player at the moment.
I get the feeling that if more people played guitar they would appreciate the more complex music rather than bands like Limp Bizkit and those other rap rockers. I know I do.(Still like Sum 41 though. Canadian, just like me!)