Well firstly the likelyhood that he did serious long-term damage to his voice on the horrendously long tours of the 80s doesn't negate him still having a decent range in his 30s. He didn't completely blow his voice on the Slippery tour so he still had a good few years of hard-rock singing left in him but by the Red bakRockers shows with Springtseen in '98 you could hear how much power and depth he'd lost. He got away with it in 2000-2001 but by the HAND tour you couldn't call him a hard rock singer anymore, although seemingly worked hard to recover some of his range by 2008 when he really started going fo those long, high notes at the end of Hallelujah.

I'd suggest the main issue was a really bad singing technique when he was younger. I'm no expert but have heard enough people compare his poor technique with the way Meat Loaf destroyed his voice in the late 70s.

Meat Loaf notoriously has had problems with his voice for the last 3 decades since singing VERY badly when he was young and wrecking his chords and his range fluctuated massively over the years. There were times when he had nothing and there were times as late as 2004 in his 50s when he could still hit his high Cs at the end of Bat. Jon struggled with Always in the early 200's and put it to bed for a while until reclaiming some of his range in the last few years.

I'd suggest that smoking has relatively little to do with his loss of power and range. I think that part of it is that Jon never had a particularly 'strong' voice to begin with, and by strong, I mean the kind of voice like Stever Tyler and Axl Rose have that proved able to withstand insane amounts of screaming, screeching and falsetto rasping and still keep going. Not to mention Axl's smoking and Steven's god know's what.

Some lucky ****ers like Bruce Dickinson and Ronnie James just never lost it and kept/keep 80% of their range and power in tact. Brian Johnson can still passably screech his way through Back in Black.

Jon's unlucky more than anything, in my opinion. Those other guys' vocal chords were just made of stronger stuff.