Hmm, interesting.
So I guess we're then talking in terms of musical structure - though, as with Woodstock, noone can deny that there's not a rock culture.
Still, I suppose everything depends on how closely you identify rock with either the culture or the musical form, and which aspects of it you emphasize.
Prog rock might not be rock at all. It certainly wasn't, if you ask the punks. Jazz-rock would be an oxymoron. Rap-rock? Other hybrids?
My own sense of 'rock' has always been a bit more slanted to some of the cultural ideas related to it, which allows it to cover a broader spectrum of music.
Going back to my beloved Fripp (

), he spoke very vividly of a huge change in rock culture which took place somewhere around 1971, and certainly by 1974. He attributed it both to the development of new massively-popular rock bands and 'greatest hits' of the post-Beatles era, which undercut the creative spirits quite a bit; and, curiously, the change from marijuana and extasy to heroin and crack. The audiences were suddenly very different, and the music they wanted was very different.
Otherwise, rock in other countries could be interesting to look at. I would argue that, for instance, the guy in my avatar - Yuri Shevchuk, of DDT - makes rock music that is relevant in a sense that no American band's music is anymore, because the social and political context they're in is very different.