So in pre-internet era music in the US, there were essentially two realms of rock music, the underground and the mainstream. The two were mostly independent of each other, and few underground bands ascended to the overground, except for a few acts like REM and the B52s.
The mainstream rock arena was segmented too, divided between hair-metal, and alternative, which was mostly a UK thing and consisted of bands like U2, The Cure, Depeche Mode, New Order etc. Unfortunately, these bands were mostly blacklisted from US rock radio, since mainstream rock stations were once again, divided into two main formats: oldies, and pop-metal.
UK alternative rock was completely shunned by most US rock stations, but these bands found success on US pop radio stations, often sandwiched between such travesties as Paula Abdul and Milli Vanilli. These bands, while not considered "true" rock music at the time, were not only getting critical acclaim, but they were starting to sell shit-tons of records. Depeche Mode's Violator album was absolutely massive, yet many mainstream rock stations refused to touch it.
At the same time, most Americans had absolutely no exposure to the punk rock. As a blue haired punker in 91, people asked me a lot questions, and what I listened too. When I said "punk rock", I may as well have said gebemekalekahi rock, as there was no reference point
At the same time, the punk/indie scene was cranking, and was composed of the most passionate and unorthodox people that were content being there and only there. That attitude and philosophy was a powder keg for creativity and expression, not to mention productivity. I don't have the numbers, but underground rock music was getting more popular with each year.
Nevermind then comes out. To people who only had exposure to radio music, it sounded foreign, not to mention dangerous. For people that had only ever listened to mainstream music, it was a definite new experience, probably akin to smoking pot/doing drugs for the first time. They weren't sure they liked it at first, but it sure was exciting. However, the album engaged all likes of people, black, white, gay, straight, through its sheer novelty, but it made them actual fans due the tremendous song writing.
What was the catalyst? How did this album get so huge so fast, without having radio stations playing it initially? TV. Specifically, MTV.
MTV had finally decided to kill the spandex clad radio star. Overnight, radio stations changed with all new formats.
So to answer your question, if Nirvana only sold 50,000 copies, what would have happened? We'd obviously still be rocking feathered mullets listening to Trixter.
Realistically, MTV would have found another band to fill in Nirvana's gap, and everything would have been essentially the same.