I wouldn't worry too much about that. All of the U2 band members have said at one point or another that if they did not feel they could give their best, if they didnt have it anymore they would hang it up. Larry Mullen was interviewed by anti-U2 Chicago newspaper critic Jim DeRegoatis or however you spell it, in 2005 and said something to the effect of: "With respect to you and your colleagues, when it is time for U2 to get the proverbial bullet in the head, we will do that ourselves, thank you very much. We would not be putting out this material if we did not think that we could keep getting better. We owe our fans this, etc, we have experimented on them they have staid with us all these years...." -a com,plete paraphrase, but google it for the exact interview, very insightful from the man who probably has the most pull and veto power in the band, Larry Mullen Jr.
Also, Bono's voice has continually gotten better since 2000 and was amazing for alot of the Vertigo tour, especially the last leg, which saw some of the best performances of U2 classics since Zoo TV. Check out Bad Auckland, One Tree Hill Auckland, Pride Japan, Streets NZ, NYD Japan, all sounded the best they have in a very long time. In addition, check out the Union Chapel gig in Nov 2007, its all on youtube. Wave of Sorrow, Desire, Stay all especially good. The most recent performances have only saw Bono's voice getting better, and that is the only variable-Larry, Edge and Adam are still the same and Adam for one has gotten much better w/ time. Bono has the control and technique part of his voice that he has mastered since having throat surgery before the elevation tour, and now the power of the Joshua Tree Rattle and Hum days is starting to come back. It is amazing to think how good they will be if this trend holds up- a 1980s powerful voice that he knows how to control and not blow out like he did in the last few Lovetown shows in 1989! Long story short, do not worry, they have too much respect for themselves and their fans to go out there if they know they will suck.
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