Very interesting, this is a Brazilian vocal coach analyzing Jon's voice (video from March'17).
Maybe some of our Brazilian/Portuguese friends will chime in and comment. I'm going to do a summary of what I understood:
First part, he talks about how vocal coaching wasn't a thing when BJ started and many singers didn't know any better, their priority was the albums.
Jon is a classic example of a singer straining to deliver outside his abilities/comfort zone when he didn't have to do it. If he'd had the right technique or stuck to a more conservative approach he'll have retained more voice.
Around the 8min mark he analyzes a recent Wanted performance (from the NA leg). He basically says what we've all seen and talked about for months: that Jon is too aware of every note and that's why he looks tense and sounds so uneven. I also understood that he's mixing all the techniques he knows to sound as best as he can, but it is counterintuitive because he's not "performing" the song (just like we've theorized before)
So according to this vocal coach, Jon's decline is basically due to singing above his capabilities/not using the right vocal techniques and he doesn't sound bad at all when he's not too worried about singing the notes, when he lets loose and acts as a frontman. Which is what happened in the last two shows, go figure
BON JOVI - O QUE HOUVE COM A VOZ DELE? - YouTube