A lot of rock/metal vocalists usually have one voice. AcDc's vocalists Bon/Brian have only one voice. Def leppard's Joe Elliot has one voice that he an make high and the same with Iron Maidens Bruce Dickenson.
How many voices over the year do you think that Rob has. Use any the songs to illistrate your point.
YGANT/LAM you have Rob' rocker voice. Painkiller,SFV/FWB have his Goblin voice. WFF/D and R/GManilishi have his croner voice. Plus he can mix it up. I think that he is a better singer at 57 than he was at 27. He does not have to sing super high all the. I love that he can mix it up.
I am a Led Zeppelin fan and I can tell you that Rob along with Aerosmiths Steven Tyler sound better now than they did when they were early 20's.