re: Would Kurt Cobain have been more successful than Eddie Vedder?

From what standpoint is Nirvana NOT a punk band? They are punk from a musical standpoint for their simple song structures, lyrical content, and love of pwoer chords (and musical screaming). But punk is more of an ethos than anything. It is defined sociologically more than musically (which is why it houses bands as diverse as the Exploited to Beat Happening to Dashboard Confessional). The basic musical rule of punk is the concept of Year Zero, and that you reject everything and start from scratch. American punk is also extremely DIY, so the method of self-recording and self-distribution, particularly before file sharing, is vitally important.

Nirvana meets a lot these criteria, and they are one of the core bands of Sub Pop. Sure, they had great pop sensibilities, but that's not a disqualifier. So did the Ramones. I'm sure they'd lose on a "who's punker than you" scale to the Melvins or Mudhoney, but no one really cares about those arguments anymore. And they were silly in the first place.