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.
The idol streamer type of vtuber? They're the successor to kpop in the West and I expect them to peak at about the same level.
Just generally replacing facecam video content on the internet with avatars? God I hope so, it'll be a nice step along the way to renomalizing anonymity in the internet and at least force entertainers to have some personality again.