As did the 80s, from Black Flag to the Flaming Lips to the Smiths.
You just said "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!)."
Hes not a strong vocalist, and Heart Shaped Box isn't some deep song and you treat it like the meanings secret, hes talked about it in interviews and in the liner notes of In Utero.
If Nirvana was so anti-mainstream they probably wouldn't have played MTV in the first place.