Rock isn't dead, it's just taking an afternoon nap like its fans do now.

Unfortunately that high range corrodes with age even for the best male singers. It's a real problem for a lot of those early metal squealers who were in their early twenties when they were rockin' it but are now... well... a lot older. As a vocalist (and someone who smoked way too much) I learned very quickly that if I wanted to be able to perform my tunes without straining and cracking as I aged I best write them to be in a lower register.
 
Paul also had that underlying raspiness/wheeziness that seems to really make a high pitch vocalist lose their range more quickly. An extreme example is Brian Johnson of AC/DC. His voice seemed to crap out for the high stuff live almost as soon as he joined the band but he covers it alright and really AC/DC is more about the guits and simple plodding beats so who cares. Axl Rose? He went downhill fast. Dickinson has held on pretty well but that stuff is so crazy and he's running around like such a looney tune dropping a couple of those ultra high wails is understandable (but he definitely covers well by just hitting another appropriate note be it an octave, fifth or whatever).
 
I think that's why a lot of metal has turned to more low end gutteral grunts and growls. Much easier to pull off live and still act like a maniac. It's a little... uh... manlier too which Mr. Stanley kind of... well... um... no.
 
God of Thunder and Dr. Love are my favorite Kiss songs... both lead by Gene.
 
Cool you got to see them.

I dig Dr. Love - but unfortunately it wasn't on the setlist. 
 
God of Thunder was, obviously. I can't help trying to imagine if Ezrin hadn't forced Paul's hand and convinced him that Gene had to sing that one. I totally get how Paul could have thought that this was his theme song. I do like his version.
 
One odd thing is that for whatever reason, I'd always imagined Stanley to be bigger than that - at least as big as I am, you know, with relatively wide shoulders. Though he's tall, he's actually built on a pretty narrow frame. There's something very teenager-esque about him. That being said, I hope I'm as fit as he seems to be when I'm his age.
 
As for the rest, I guess it'd be bad news for the youth if a 62 years old could still sing the way he did in his 20s and 30s. IMHO, he kept his voice much longer than many other classic rock singers - his performance on Unplugged (mid 90s) was pretty impressive. 
 
But eventually, a voice dries up - that vibrato slows down and becomes exaggerated and no matter how hard you try, those notes just won't come out.

Glad you had a good time, Rain!