Sometimes age can do very interesting things to someone's voice. I love Bob Dylan's voice on his last album (Love and Theft). It is extremely cracked and, technically speaking, awful, but I am just transfixed when listening to it. Especially considering how great the lyrics are.
So I am not worried about Bono's voice, it would get boring if his voice was the same on every album.
So I am not worried about Bono's voice, it would get boring if his voice was the same on every album.