Yeah, but as far as Grunge was founded as an opposition to the rock mainstream
I think the more appropriate phrase would be "as an alternative to.." because 'grunge' was a marketing ploy which existed solely to sell records.
THIS. Like with any music scene, the whole thing was an organic process that became categorized after the fact. It wasn't some organized attempt to shit on earlier music like some punk artists did. I mean, hell, the guys from Pearl Jam pretty much openly jerk off to Neil Young on stage. Jerry Cantrell of Alice in Chains is one of the few grunge artists who gets even a shred of respect from the metal community, although it's more for his respect and deference to the genre than for his contributions. That said, songs like
We Die Young have as much in common with Pantera & Black Sabbath dirges as they do with anything Cobain did. Soundgarden was basically noise rock crossed with Led Zeppelin. Frankly, if there was any contempt for the rock scene, it seemed aimed at the lyrical content and lunkheaded macho idiocy espoused by groups like Motley Crue, not at rock or even metal in general.
So yeah, I'd say "grunge" lacks legs, but only because it's a narrow, artificial label that segregates grunge from the wider angry-young-men-with-guitars style of music for no good reason.
P.S.
I left out talk of Nirvana because Kurt Cobain was a miserable young man strung out on heroin and dumb enough to marry Courtney Love. While he showed contempt for his contemporaries on several occasions, I don't really think he had any real plans in mind when he first picked up a guitar. Even if he did, it's rather a stretch to apply his mindset to all the other bands that grew out of the same scene.