Did you know that (English) academic papers about vtubers were written as early as April 2019?
Well, now you do.
The paper in question.
It mentions Kizuna Ai and the four heavenly kings, as is appropriate for the era, and also singles out Nijisanji, as well as mentioning some smaller corps of the era, such as .LIVE. No Cover/Hololive, however.
There is a fair bit of gender politics in the paper (but not necessarily as you may imagine; the babiniku concept and Japanese views on femininity are viewed and partially criticised from a largely feminist perspective, the paper leans more gender critical than trans), which I'm pretty sure was added to buff the wordcount.
What I found quite interesting is the paper's discussion of virtual idols (apparently dating to the mid 90s), and its prediction that vtubers may be the second coming of them (as previously noted, the paper doesn't mention Cover or Hololive; don't know if Kizuna Ai had already started doing idol content by then).
And what I also found interesting was its discussion of slice of life media featuring cute girls, the increasing disappearance of plot or narrative in favor of pure character in Japanese media, of which it concluded that vtubers were the ultmate expression.
Right on the money on those last two points, I'd say, and the idol thing was a pretty impressive prediction, IMO.