Well, the guy basically reinvented the way we look at amps, and even guitars to a great extent through his own personal experimentation. Let's not try and downplay this.
The Peavey 5150 was around for 12 years, same so far as his EVH brand through Fender. I've seen other artists go through endorsements with much greater frequency.
I think it hasn't helped that many who've done business with Ed cite his being an impossible asshole to work with.