The West, USA in particular, does not as a rule take boy bands/girl gets seriously. They never really have. This type of pop music is considered the realm of teenagers, particularly teenage girls. That's not new and that's not unique.

If they get a Grammy nod (and I don't mean this as a diss to BTS, but rather one to the Grammy Academy) it will be bc the Grammy academy is trying to stay relevant. All the major academies have been trying to not seem like old fuddy-duddies for awhile, and so try to cater toward what they think young ppl might like. They see that teenagers will watch an entire award show to see if BTS will win and the academy wants ratings. (Yes, I'm pretty cynical about this but I generally think awards for arts are kind of bs. I mean, the Oscars is considered fairly respectable and look at the terrible actors that have won that? Again that's not a diss on BTS, that's a statement that these awards have nothing to do with quality and everything to do with politics and marketing.) Additionally the Grammys have so many categories that they have often been perceived as meaningless awards anyway. Like, there's been jokes to that effect on the Simpsons, that's how mainstream that belief is.

BTS, and kpop generally, remain incredibly niche. I'm from smack dab in the middle of the country and I've never once heard anything irl about BTS or kpop generally, I've never heard it on the radio, never seen an album in a store, etc. If they're nominated/win, it's not that most ppl will react negatively, it's that they won't react at all.