Something to consider. Vtubers live off internet fame. Some have life experience and a liable to raise hell if you try to fuck with them (e.g. hags), but rather a lot don't. And we know that Niji requires PLs to be shut down (Holo, Phase Connect or Idol by contrast, don't), which I imagine is already filtering out some of the more outspoken candidates in favour of hopefuls who are ready to throw everything away for their big break. Hopefuls who willingly burn their own bridges to leave them without an alternative.
Obviously, Sayu was more outspoken than Niji expected, and so was Selen, but I imagine their legal department had a lot more experience with livers that'd already been browbeaten into accepting their position after sacrificing their alternatives, unwilling to put up a fight. And Nijisanji's hiring conditions facilitate exactly that kind of attitude. It's what they're looking for. It doesn't want creativity and grand projects and talent initiative. Talents that are like that cost money and demand effort, and that's just not part of Nijisanji's low cost business model.
Nijisanji only wants you to shut up and stream.
Big question for me is how Niji is going to course correct without pissing off the shareholders.
As you say, Nijisanji already has minimal expenses, there is hardly anything to cut. And we know from Niji's financial reports proudly noting their minimal expenses and Cover's Q&As featuring Yagoo beating back vultures complaining about excessive expenses and talent renumeration that the average shareholder knows little about the industry but really likes cutting expenses.
You might get a situation where Niji knows how to fix the problem (rainbow goes here), but attempting to do so might lead to a shareholder revolt.
That's not necessarily a problem if Niji owns enough of its own stock to tell the other shareholders "Fuck you, lol", but doing so would tank the stock.
That is also not necessarily a problem if Niji can just sit on its shares and wait until problems are fixed and line goes up again, but if Niji relies on occasionally selling some shares to deal with outstanding debts and similar (as is more or less the norm), then it becomes a potential death sentence.
Also it'd tank the value if Nijisanji plans to issue more shares in the future.
This isn't to say that Niji is doomed, since (Gundou and Chihiro notwithstanding) JP is trucking along, and we really don't have a picture of how affected EN is in the long term, either. Just speculation.
But while it may or may not take it, there is now a visible path to doom and Niji might potentially be quite limited in its available means to avoid it.
Though since Holo's ability to grow is IMO limited by its Idol/event niche, I don't think Nijisanji is likely to die unless a competitor fitting the same casual streaming niche pops up.