We won't know until it happens. After all, we didn't really notice Nirvana until Nevermind made it big.
There will always be those of us over a certain age that will say 'There will never be another XXXXX' and there is a grain of truth in it, but that is because the music industry has changed so much more in this decade than in the combined 4 that proceeded it - now don't get me wrong, I am not saying MUSIC has changed more in that time, but the MUSIC INDUSTRY has and that will make a huge difference to what bands get the chance to break out and change the world. Bands in the past got big by touring and building support. They got heavy airplay on radio and TV. But live music is different now with 'nostalgia' acts clogging up the arenas and promoters and venues pricing rising bands out of club circuits in favour of tribute bands for people that don't care about music and just want a night out.
As for airplay, there are so many more radio stations these days that they can afford to specialise meaning that the likes of Nirvana would probably be stuck on specialist stations while Simon Cowell decides what goes on the mainstream channels. Meanwhile MTV haven't played a music video in years and are more concerned with reality shows.
So although there has to be another singer out there (or at least that will be born sometime soon) that can wail like Robert Plant or play guitar like Hendrix, it is just that I fear we will never get to hear them. Christ, I hope I am wrong.