If you mean U2 becoming like R.E.M, Depeche Mode, Pearl Jam, Radiohead...etc, I don't think it would be a bad thing.
The question is would U2 be happy selling ~3mln copies of each future album? and becoming more of a "cult" band...
I think relevant is a complicated definition these days. REM is always brillant, DM in the nineties chose to be a cult band, Radiohead is still big. But the kind of relevance these bands used to have was another kind. Remember when Pearl Jam shared all the news with Nirvana as hot epic grunge band?
But now what you can call relevance is an act that is in the news. Britney Spears sold more than a million coppies with that ultra crap music she does. I'm saying that real music is non important in pop scene. And i'm talking about "pop scene" not rock...
Even pop acts had to deliver pop gems over the years. Madonna, George Micheal, Spice Girls, Phill Collins, Alanis, Billy Idol and all others acts were top class pop acts. Their music had to contain that plus that make pop music listenable at least.
But after the Backstreet Boys/N'sync Britney and Chritina revolution pop music descend to their all time low levels of quality. I can't remember any good pop gem from any of those acts, nothing close to Like a Virgin or Dancing with my self...
You don't have to have a good song(for pop standarts) to be a big act.
Britney Spears is argueably the biggest artist in the world without having any good pop song in her catalogue. We are living in dark days for music.
We have some nice rock bands in the scene, as Coldplay being the biggest of them and the biggest selling artist of last year, even with the super boring, U2-pale carbon copy abum Viva la Vida. You see a pattern here...
So if in this pop scene music matters the least is natural that rock will not be relevant for at least a couple of years, because music itself lost its relevance.