How much was Nirvana really responsible for the downfall of glam metal?

I've discussed Nirvana and the rise of grunge on AskHistorians before - /u/scarlet_sage has kindly linked to a previous answer elsewhere on this thread, and I've discussed it here too - especially see my replies to /u/subs-man in that thread.

Genres generally die, of course, because a lot of the young people buying the records grow up and move on to other things in their lives, and get replaced as music buyers by younger people who want new things. But this process is not instant. To take an equivalent genre, 1970s British glam rock: T.Rex had a run of British top 10 hits from 1970-1973, while Slade had a run of top 10 hits from 1971-1974. Gary Glitter's run of top 10 lasted from about 1972-1975. Gary Glitter still having top 10 hits in 1975 was a little anachronistic, even (and deeply unsettling, considering the age of his audience and his child sexual abuse charges), but not every fan immediately stops being a fan just because there's a new musical sound. Of course, not every glam rocker had hung up their sequinned boots in 1975. David Bowie was originally a glam rocker extraordinaire, and commercially he very obviously outlasted glam rock - and punk - and new wave (as did another glam rock band, Queen).

The 'death' of the 1980s glam metal worked in exactly the same way - you wouldn't expect the fans to instantly hang up their perming machines and put away their leopard-skin leotards in favour of bedhead and flannelettes. So it's not at all surprising that Def Leppard or Guns 'n' Roses or Bon Jovi continued to have careers after 1991. The smaller (if still popular) late 1980s glam metal bands, on the other hand, do seem to feel that they were screwed over by Seattle. To quote from the oral history I Want My MTV:

KIP WINGER [of Winger]: I was talking to Rick Krim on the phone and he said, “You haven’t heard of Nirvana?” So I watched “Smells Like Teen Spirit” and I thought, All right, we’re finished. We all knew it. It was obvious. There was no “Won’t you play our video?” MTV wiped the slate.

JANI LANE [of Warrant]: My manager and I flew to New York to say, “Please play our new video, ‘Bitter Pill.’ Just give it a shot.” And Rick Krim said, “I can’t do it.” It was a real blow. I went home thinking, My career’s over.

Nirvana were, essentially, the commercial vanguard of alternative rock. This isn't to say that they were the biggest act, and they certainly weren’t the first alternative act - a series of alternative/indie bands had percolated towards the mainstream in the late 1980s, with indie bands like Sonic Youth, The Replacements and R.E.M. signing to major labels and R.E.M. having some real mainstream success in particular.

And a stream of funk-influenced punk-ish hard rock broke through to the mainstream before Nirvana, with the likes of Red Hot Chili Peppers, Jane’s Addiction, and Faith No More providing a cooler alternative to hair metal; it was weird enough to be played on MTV’s late night alternative music showcase 120 Minutes alongside stuff by Morrissey or New Order or the Cocteau Twins, but it was hard rock enough to appeal to teenage boys who would have disliked Morrissey’s cultured jangly pop.

But Nirvana absolutely were the act who made record companies think that alternative rock was commercial manna from heaven - it was simply so implausible that something as hard and uncompromising as ‘Smells Like Teen Spirit’ was in the Top 10, unless it represented a new style. As a result, subsequent bands were very often marketed according to the formula that had first worked with Nirvana (the teenaged Australian band Silverchair, who hit it big in late 1994/early 1995 with 'Tomorrow' were colloquially known as 'Nirvana in Pyjamas' and for good reason). 'Smells Like Teen Spirit' debuted into the Billboard Hot 100 Singles chart on December 7th 1991, at a point when singles by Guns 'n' Roses, Van Halen and Extreme were higher in the charts, and when the second-most alternative thing in the Hot 100 was probably 'Mysterious Ways' by U2 (it may be that part of why 'Smells Like Teen Spirit' isn't the highest ranking alternative rock song on the 1992 charts is that a significant part of its sales occurred in December 1991).

It was Nirvana's breakthrough - on MTV and in the charts - that created a clear commercial pop music market for alternative rock as a genre. As you say, the Red Hot Chili Peppers and Pearl Jam exploited more successfully than Nirvana did, but it pays to remember that part of Nirvana's mystique was their rejection of 'playing the corporate rock game' and making the kinds of compromises that might sell records to middle America.

Pearl Jam and the Red Hot Chili Peppers had less qualms about this kind of thing in 1992, and their sound at the time was ultimately more commercial - for instance, 'Alive' has just enough touches of glam metal to have sounded familiar in 1992; but it sounds so over-the-top today that Pearl Jam released a remixed version of Ten, Ten Redux, in 2009. This remix largely got rid of the cavernous reverb on the album, a production sound that's often associated with glam metal, and not with grunge.

And of course, part of why Nirvana were not as successful as Pearl Jam during Cobain's life was that their 1993 album In Utero had very little pop sheen to it, with a particularly raw sound thanks to engineer Steve Albini - Nirvana were actively rejecting the idea of trying to make a follow-up hit that would be even bigger than Nevermind, the way that a commercial rock band is supposed to.

By the time that Nevermind got to #1 as an album in early January 1992, other alternative acts were starting to make their presence known - RHCP's Blood Sugar Sex Majik was #35, Soundgarden's Badmotorfinger was #88, Nine Inch Nails' Pretty Hate Machine was #94, Pearl Jam's Ten was #143, Alice In Chains' Facelift was #163. And all of these acts rose in the charts that week, pulled in Nirvana's wake; RHCP and Pearl Jam would soon be in the Top 10 along with Nevermind, while the slow-burning success of Badmotorfinger, Pretty Hate Machine, and Facelift across 1992 (all three albums were still in the Top 200 albums a year later, in January 1993) would translate into very popular, prominent, follow-up albums for those acts in 1993-1994. People liked what they heard of Nirvana, in other words, and record companies scrambled to promote the acts on their books that were a little bit like Nirvana - in some cases incredibly successfully (Ten was still in the Top 10 a year after Nevermind got to #1).

Nirvana had also kickstarted a (paradoxically) incredibly popular genre where looking like you wanted commercial success was incredibly uncool; Nirvana may not have sold as many records as Pearl Jam, but because they set the rules of alternative rock, and because they at least looked like they didn't give a shit if they were popular, they were considerably cooler than Pearl Jam. And in 1992, that mattered, because a Nirvana-esque coolness was implicit in the ideology of grunge (as any number of Daria episodes on MTV, for example, satirised).

Cobain also very prominently accused Pearl Jam of being bandwagon jumpers, in the cover story of the April 1992 Rolling Stone:

Although Cobain is thrilled when underground bands infiltrate the mainstream charts, he's outraged by others who are riding the coattails of the alternative boom. His favorite target is Pearl Jam, also from Seattle, which he accused of "corporate, alternative and cock-rock fusion'' in a recent Musician magazine interview. "Every article I see written about them, they mention us, and they're baiting that fact,'' says Cobain, sitting up cross-legged on the bed. "I would love to be erased from my association with that band and other corporate bands like the Nymphs and a few other felons. I do feel a duty to warn the kids of false music that's claiming to be underground or alternative. They're jumping on the alternative bandwagon.

This opinion about Pearl Jam was initially shared by quite a few cool indie types with newspaper columns. The Chicago critic Jim DeRogatis says in Milk It: Collected Musings On The Alternative Rock Explosion Of The 1990s that 'I initially found Pearl Jam's debut to be histrionic and melodramatic, and those traits grew from annoying to grating as the singles 'Even Flow' and "Jeremy' became ubiquitous on modern-rock radio'.

Anyway, this kind of thing (along with Nirvana being the band to cut through first) meant that, while Pearl Jam might have sold more records, they didn't have as much cultural capital as Nirvana in 1992-1994; Nirvana and the Red Hot Chili Peppers were on the cover of the Rolling Stone in 1992, and Pearl Jam...weren't.