By 'mainstream', I mean that Vtubers like in Japan would be on the morning talk shows, be featured in advertisements on TV, have shows about V-Tubers on TV (there is a light novel with an anime adaptation called VTuber Legend: How I Went Viral After Forgetting to Turn Off My Stream), collaborations with V-Tubers and sports teams are a common occurrence, and while we have had some of these things in the West, it's not as pervasive as it is in Japan.
In my opinion with Western media being continually ass, it's only a matter of time.
God I hope not. You already have "normies" trying to police the artwork of vtubers to ensure they're not lolis and various other social media shenanigans. Making it more mainstream and more popular will just make it worse, just like anime. As South Park put it jokingly "make it lame and gay" but this time seriously.