The Who quit in 1983....only to come back and tour again later. The Rolling Stones nearly broke up after 1983 because a rift between Mick and Keith. So in a sense had the Stones stopped....the reins would have fell to none other than U2. U2 would have taken the reins, but created TUF, with hits Pride, Bad, and ASOH.
So, if U2 want to quit, they can pass the reins onto say, Coldplay. They've got 4 albums out, each one widely respected.* Viva has controversey, because it alienated some fans wondering what did they do? This is the same instance happened with U2 fans when Achtung came out. I'm not saying Coldplay are copying U2, but Coldplay could be the next big band.
But in my case, U2 has this album, then their last one. Just remember the words from Roger Daltrey, lived out by Keith Moon, "I hope I die before I get old". In other words, to U2, don't let your act grow old.
