I do think pop metal or glam metal had very much run its course by 1991. Initially you had one crop of bands that felt fresh, exciting, and while not Harvard material, were certainly a fun bit of escapism. Then you more and more batches of these bands, each worse than the last. People got fed up, they got sick of it. And people forget, alternative bands or even just throwbacks like GnR and The Black Crowes were already making a mark before Nevermind. R.E.M. were getting popular, Chili Peppers, Faith No More, Jane's Addiction, Soundgarden to a degree, Nirvana just broke the final barriers. But yes, the genre did itself in.