Okay, first of all Nirvana never claimed to be the first grunge band. I don't know many people who do. Second, Nirvana brought grunge into mainstreem, a significant blow to the Bubble Gum pop, Hair Metal, Arena Rock and dance pop that dominated back then. Although pop music has (in the end) won the war, Nirvana (helped) get us out of the crazy sounRAB of the 80's.
Thirdly, not many of the fans of the original grunge sound like Nickelback or post-grunge of any kind (I know I sure don't!).
Last, Cobain had a unique voice all his own. Compared to many artist of today, he was extremely unique. His lyrics are often cryptic, so boneheaRAB can't understand them (Something he himself admitted in the song "In Bloom").
So don't you go downing my favorite band like that!