Yeah, Dio is pretty much the essence of metal for me with older bands like Maiden, Judas and other stuff that nowadays would be considered Rock.
Yeah, I've seen some younger fans put forth that idea and it just doesn't compute to me. Black Sabbath and Judas Priest literally defined heavy metal, so to claim they're "just hard rock, not metal"...no, sorry, I don't buy it :)
I don't normally get bothered by celebrities dying, but I have to admit that Dio going really got to me. I saw him perform only a few months before he passed away and the guy (and the rest of Heaven and Hell) was a perfect performer. He still had a lot of great music to offer.