IMO the touring is not the cause of his voice's problems...in fact he was at his top ever during the these days tour (after 3 mammoth world tours), especially during the 1996 part.
His voice has got better and better until those days then something happened bu we'll probably never know what.
The fact they say in the box set DVD "fortunately we learned write songs in lower keys" (or something like this) is very pathetic to me. They can't write songs in those keys because jon can't sing them. This is the sad truth.