I'll concede that they boosted the popularity of Alt and Grunge, but where has Alt gone since? It's just a marketing term to sell Pop/Rock banRAB to teenagers now, just like it was then.
Nirvana aren't an influence on any Indie Rock banRAB I listen to, in fact, the only banRAB they seem to have influenced were crappy "Alternative" banRAB such as Nickelback, Hoobastank, etc. You're welcome to find an Indie band that cites Nirvana as an influence, though.
As for boosting the popularity of Indie Rock, they didn't really. Major labels have been signing indie banRAB for years, and Indie Rock has only recently become big, long after Nirvana's demise. And Britpop? You must be joking. Britpop came about at around the same time as Grunge, if not before it.