The band almost called it quits after Pop. Bono said that when they went in to record ATYCLB, they were determined to learn from their mistakes in making Pop, but that if the next album were no better, they would break up. That's history, but I suppose history could repeat itself. As Saracene said, it's hard to know when to quit until you do a few crap albums.
On the other hand, Adam has said he sees it as totally appropriate that as the band members age, and their core audience dwindles, they continue to record and play smaller gigs. He said it would be very much like what Dylan is doing; the point is to keep the songs alive by continuing to perform them.
Personally, I might have a hard time accepting the group as anything less than the world's biggest and greatest rock band, simply because the way they put themselves forth is so much larger than life. But if the quality of their work holds up, I think they should keep making it.