I think this has been interpreted in two ways, whether the band can physically do it or creatively do it.
I think physically they could keep playing live for a few more years without a problem, as musicians I don't think they would slow down. With the exception of Larry the show isn't that physical for them as say Metallica as mentioned earlier. How Bonos voice and presence will hold up over the next few years though remains to be seen and that will just come down to age same as it would for anyone. They famously said the only way out of U2 is in a box so I don't think they intend to stop anytime soon, there are financial gains to be made remaining active but I don't think that would solely motivate them.
Creatively I think that is really hard to predict, every U2 album is a leap off the edge (no pun intended) for everyone and I think it really can go either way these days. I don't think I'll ever abandon them but I do find myself wincing a bit more listening to some of newer songs and Bono's soundbites. A friend of mine, another huge fan, were talking about it recently and I said I think U2 have entered their second Rattle & Hum phase, the huge rockstar mega band, collaborating with other 'influential' artists or trying too hard to find a new audience if you will when the old one is perfectly happy and can easily sustain them.
U2 look more like rockstars now then ever, leather jackets and trousers, studded belts and playing music like it, almost an old band trying to sound young, whereas if you look at TUF, JT or AB it sounds like a young band sounding wiser than their years.
I personally think they have more in them musically but I think they need to almost pair things back and do it for themselves. Just being a band without trying to be the world's band.
But it is hard to imagine them not being around though and for all the flack they get, especially here at home, people won't realise how important they were until they aren't around anymore.