nah. most late '60s/early '70s bands were stoned off their faces and trying to make as wild of a trip as possible, which is no more highbrow of a goal than any of the ones you're railing against. if anything, that era was probably more homogenous and unthinkingly trendy than extreme metal is, though it had its fair share of gems. i'm resisting the temptation to laugh you out of the room for even talking about 'late '60s traditional metal bands' at all--i'm pretty loose about genres but c'mon, man.
anyway, forget the sounding nasty/ugly thing. to me, what defines metal as a genre from the beginning is the way it confronts the aspects of reality that popular music/culture buries under the surface. black metal and death metal are, again, the logical endpoints of this. i'm not saying they're better (i'm probably more of an '80s trad man than anything else when pushed), but they're the natural evolution thematically. the aesthetic just grows out of that. black sabbath were trying to sound darker than everybody else, trying to confront uglier truths. this isn't at odds with trying to make good music - indeed, they felt that music had to be this way TO be good. darkthrone and the rest are no fucking different, they just took it to greater extremes.