My two cents - I used to blame it on smoking for Jon, but I don't anymore. It played a part, sure, but I've seen too many singers that have sustained their voices in a good-great shape despite being heavy smokers.

It's never healthy to push your vocal chords to the top of your range and that's what Jon has done his entire career - in studio (120% effort) and on stage (100% effort). Good singers sing in their comfort zone most of the time and maybe occasionally showcase their voice once per concert/album or so. But Jon always recorded in the highest key humanly possible and it was extremely taxing for him to try to replicate it (even half step down) during live performances given their heavy touring schedule. Even in the 90s, when we consider Jon's voice to be great (righftully so), he was still straining it every night during some songs - remember, he did In These Arms and Dry County in original key in 1992; Always and Saturday Night in original key in 1994; Something To Believe In half-step down, but still with all the high notes! then he sometimes did With A Little Help From My Friends - Crazy! And in the 2008-2011, like bonjovi90 said, he pushed himself to give us some highly-demanding stuff and the decline was already evident in 2011. It seemed to me at the time like it was just fatigue (the end of the long tour), but it obviously proved to be permanent. And sadly, his voice will keep getting worse and worse, because right now he strains the shit out of his chords just so he could attempt to reach the right key in what used to be easy stuff, like Bad Name, Wanted, IML, etc., while in studio stuff like Roller Coaster, Labor Of Love, Scars, title track, etc. are all things that may not seem like challenging, but it's still above the "sustainability line" for him and he still has to push hard to nail it live.