Nirvana were dead in the water once Nevermind was released. Over produced, mainstream version of what Bleach was...
It would have been interesting to see what happened if Cobain had lived. In utero was woeful, product of someone no longer giving a sh*t. Would Nirvana have morphed into the musical style of Foo Fighters? Doubtful - probably would have imploded.
It's clear with hindsight that Grohl was head and shoulders far more talented than anyone else in Nirvana.