I initially battled to get used to his deeper voice which started occurring from the OES tour but I grew to like it.
For me the big factor here is the effect of the gruelling touring on his voice:
1) You will notice his voice is always decent on the studio albums, this as he is going in with a "fresh" rejuvenated and rested voice.
2) Listen to how his voice takes strain over the course of a tour, starting with OES, the 1991 shows were smooth, but if you take a listen to On The Night and Basel Switzerland from 1992, you will see what I mean. This same feature applies to all the tours, just sample any early dates vs. later dates on any tour. I remember how good it sounded in 2005 the first 2 shows in South Africa when I was there live and then towards the end of the tour.
3) His voice was really deep from about 1996, Golden Heart and in particular the STP tour in 2001 then he stopped smoking and it improved.
So I think there is a distinction between a deep pleasant voice, on all his albums and the early tour shows vs. a strained/damaged voice later on in the tours which is much harder on the ear.