When punk arrived in the mid-70s, it seemed to mark an unequivocal line in the sand. Here, punk said, is where everything changes ... you can either go with us into the future, or you can listen to oldies the rest of your lives. Countless baby boomers did the latter, meanwhile buying lots of Tom Petty albums to prove they were still cool. The rest of us thought we were moving to the future; it took a few years to understand that punk wasn't the beginning of a new world but only the end of the old one. Only the bastardized form of punk, "new wave," made an impact on the charts ... bands like Talking Heads were revered, and artists like Elvis Costello began careers that refuse to go away, 25 years later.
That line in the sand was eventually drawn: it was rap music, and the hip-hop that subsequently emerged, which truly changed pop music into before-and-after, where "before" was rock and roll. Punk was influential, but it wasn't generally crucial in the marketplace, with the possible exception of later Clash. To be sure, some of the greatest rock and roll music of all time was 70s-based punk rock, but it's hard to argue that X-Ray Spex had as much cultural impact as Run-D.M.C..
Punk evolved into "alternative rock." Some great bands made some great records (I've always been partial to Husker Du, who managed to squeak into the Rolling Stone Top 500 at #495). "Alternative" might have been a problematic descriptor ... "indie" wasn't a lot better ... but as long as bands like the Huskers were avoiding the top of the charts like they had the plague, "alternative" was accurate enough.
And then came Nevermind. Nirvana's classic album rose to the top of the charts; in a great example of accidental symbolism, they pushed Michael Jackson from #1 in the process. There are as many explanations for the rise of Nevermind as there are people who bought the album (close to 8 million, I believe): the way charts were calculated changed to the advantage of indie rockers, the "better" sound provided by Nirvana's new "major" label made their music more accessible, "Smells Like Teen Spirit" was a great video, or the simplest explanation of all: Nevermind was one of the greatest albums ever made.
Whatever the explanation, with Nevermind punk finally became not just an artistic/cultural influence, but a serious force in the marketplace. This pissed off a lot of people, apparently including Kurt Cobain ... the snobbish insistence on the primacy of "indie" art over the mainstream has often been crushing to the work of the actual artists in question (of course, forcing mainstream values onto independent artists is at least as crushing, but indie conspiracy theorists tend to find the mainstream imposing on artists far more often than actually happens ... Nevermind is a better album than Bleach, just as Husker Du's major-label debut, Candy Apple Grey, was indistinguishable from Flip Your Wig except the indie release had crappier sound). In the case of Nirvana, this resulted in a followup album, In Utero, that was a horrified version of Nevermind, peppered with songs titles like "Dumb" and "Rape Me" and "All Apologies."
With the death of Kurt Cobain, Nevermind and pretty much the entire career of Nirvana becomes more metaphor than art. Which is unfortunate, for Nevermind remains one of the great albums, In Utero ain't far off, and the posthumous live releases Unplugged and From the Muddy Banks of the Wishkah offer numerous pleasures. Because they broke on the charts, because they then "represented" alt/indie rock, because Kurt killed himself ... for all those reasons and more, people think of Nirvana as symbol. It would be a shame if the music became secondary.
Acclaimed Music, which collates critical opinion from a large variety of sources, lists Nevermind as the most-acclaimed album of the 1990s. No subsequent album has been as honored. In fact, Nevermind is #4 on the all-time Acclaimed Music list, behind the Beach Boys and two Beatle albums. That seems fair enough to me: it's a brilliant album, full of great songs and excellent playing, that is also a cultural milestone. The RS 500 list has Nevermind at #17, the first album from the 90s (and the only one for quite awhile), with In Utero at #439 and Unplugged a bit high at #311. I might personally opt for Sleater-Kinney (who don't show up at all on the RS500), but it's hard to argue with Nirvana and Nevermind as the best alternative rock has offered in the past couple of decades.