Unholy fusion really is a perfect way to phrase it. I don't know if it is some stroke of genius or just plain luck that vtubing ended up bringing the japanese idol culture to the mainstream in western internet, but it makes complete sense given that it takes the most subjective appeal of idol culture in japan, which is the idols being japanese, to something that does away with that by using a more universally acceptable anime visual. I don't know how much being an actual pseudo idol agency has to do with the popularity, but if i had to guess its probably related to how detached the vtubers are from being actual people, specially in hololive's case.
And on that point, that is the exact reason i do not really enjoy hololive, but occasionally enjoy indie/vshoujo vtubers. Hololive reeks of the exact same reason i never enjoyed idol stuff, which is being so completely parasocial with its viewers that it barely feels like you are watching a streamer, and more like you are watching a person playing a role of an anime character being a streamer.