Eh, I guess it boils down to how we define terms like "instant" vs "gradual". I mean, did the moment that Nirvana's "Teen Spirit" break big all sales of all Hair albums instantly drop to zero? No, of course not. So in that sense the impact wasn't instantaneous. But my recollection was that it happened pretty quick, quick enough for pretty much everyone to describe it as such at the time. My recollection is that after Nirvana almost overnight it became very uncool to like bands like Motley Crue, Poison, and Warrant. And not just from the perspective of "alternative" fans that always despised Hair acts. Their own fan bases, which had numbered in the millions, largely evaporated. And this wasn't just reflected in the charts, it washed over the entire rock music scene culturally as well, it was publicly commented on. IOW's, the notion that Nirvana and grunge wiped out Hair isn't a retrospective thing, it's how it was felt at the time, in 1991-1992. And apparently it wasn't felt just in the rock world. Here's Jay-Z, recalling how the emergence of Nirvana impacted Hip-Hop's sense of itself as a rising cultural force in the early 90s (from a SPIN magazine article, 10/5/12): “First we got to go back to before grunge and why grunge happened,” reasons Jay. “‘Hair bands’ dominated the airwaves and rock became more about looks than about actual substance and what it stood for—the rebellious spirit of youth….That’s why ‘Teen Spirit’ rang so loud because it was right on point with how everyone felt, you know what I’m saying?” Jay-Z then goes on to say that grunge actually stalled the rise of hip-hop in popular culture. “It was weird because hip-hop was becoming this force, then grunge music stopped it for one second, ya know?” he says. “Those ‘hair bands’ were too easy for us to take out; when Kurt Cobain came with that statement it was like, ‘We got to wait awhile.'” You sound like you were around for the shift as well, so perhaps it's just a case where we have different perspectives.