Nirvana was hardly the first grunge band, as I think we've established. They were, however, the first to get any major coverage, and they were certainly the first "pop" grunge band. They revolutionized music: unless you think enthralling an entire generation isn't revolutionary. They took Michael Jackson out of the number one spot on the charts.
Whether or not you even LIKE Nirvana, that says something about their music. They were revolutionary because they weren't like the rest of the popular bands back then.
(BTW, someone mentioned Pearl Jam... Ten did outsell Nevermind in the end, and they were more popular than Nirvana around the time of Kurt's death).