When you put out bad records (and Pop, How to Dismantle an Atomic Bomb, and No Line on the Horizon certainly qualify as bad albums for U2), you drift into irrelevance.
Here's the real reason why U2 are irrelevant. Five years between albums? Just go away already.
U2 have released exactly four albums since 1997 (Pop, All That You Can't Leave Behind (2000), How to Dismantle an Atomic Bomb (2005), No Line on the Horizon (2009)). I'm not counting the disastrous Million Dollar Hotel project--that would be unfair. That's absolutely pathetic in terms of artistic achievement. This reveals a sense of entitlement and self-indulgence that can't be explained simply by touring endlessly and grossing tens of millions of dollars by playing every stadium in the world. 1980-84 gave us the first four U2 albums; that's how long they take between albums now.
I'm probably in the minority, but I think they should have joined R.E.M. and packed it in. Am I excited about their new album? No, and who else is? It will have a bombastic single, moments of polished brilliance, and then it will sink into oblivion. What's the point of it all anymore?