I don't know about that. Some singers do have a fine voice in their 50's - and those are inherently better vocalists. Even Bono said that he's, at best, a good vocalist, not a great one. I agree. It's the passion he brings - along with some sky-rocketing notes - that made me a huge fan of his vocals. In terms of passion, Bono wins almost always.
However, when I look at other singers around that age (including a few that have since passed away), they can be great - in a very narrow range. If Bono stayed within a certain range, he'd be fine. But that might limit his passion and limits what U2 can do.
The loss of his range and power is clearly due to smoking, IMO. The TUF Bono was young and rarely had issues. By JT, though, some nights he simply wasn't able to hit notes or a falsetto. The effects became a bit more pronounced for Love Town when shows were cancelled. He was smoking and pushing his voice hard. For AB and ZOO TV, Bono's style changed, allowing the appearance of solid vocals, but it's the change in style, less straining on AB songs (where a few were almost "spoken") that helped. Plus, there were some ZOO TV shows where clearly his voice is failing (I recall a Fox TV special and he sounded horrible on "Mysterious Ways" - his voice was gone by then). By PopMart, the breakdown of his voice due to smoking was more obvious. With Elevation, there were moments of recovery and moments of some of his worst vocals ever. Either he quit smoking or decreased it tremendously, because this current Bono is the best I've heard since ZOO TV. Is it as good as that Bono? No, but it is close. And at times, he has the power to match the JT/Lovetown era Bono (see "Miss Sarajevo" performances, especially on the "Vertigo" tour).
Lesson? Sorry to all the smokers out there,
but don't smoke, especially if you are a vocalist.