The problem with that logic is the riffs suffer, and they suffer horribly. When you hear a riff and love a riff it becomes part of your DNA. Speed changes don't bother me so much and it can actually become beneficial to the song (see Unleashed In The East for a primary example). Key and melody changes however, are unignorable. Melody and key is the essence of song, and the problem with key shifting is that it hits some truly atrocious notes and the songs become unlistenable. 2112 sounds like a fucking mess when it's downtuned. Same with The Mob Rules. I was actually happy I missed that song when I went to see Heaven & Hell because that riff sounds like its being raped. and it's not just tuning down. I was watching a 78 Sabbath show and rather than having guitar techs doing their job and tuning separate guitars for the appropriate songs, they just played everything in standard. Children of The Grave, my all time favorite Sabbath song, was unlistenable in this key.
Also, if voices go through so many goddamn changes, then why the hell was Rob Halford able to nail every single note on the Judas Priest songs on Live Insurrection, when he was fucking 50?
Like I said, either you play the song in the right key, or you stop touring, it's as simple is that (or in Geddy Lee's case, he can just sing that song in a pirate voice
) Honestly, I could care less how singers sound in old age, but when you fuck with the riffs, that's capital punishment grounds.