Something that I don't think people are putting enough thought into is that Hololive is a challenging job that requires a stupid time investment and everyone who is leaving has been there for a significant amount of time. Work like this burns people out, even if you enjoy it, and sooner or later people will need to move on.
The video games industry is a similar high intensity industry where the average time someone spends in it, total, is 5 years. Even in well run companies, the overtime and crunch wears out its welcome regardless of how much it may have been your dream job throughout your secondary school and university years. I hit 5 years and had to bail out because what was a tolerable time investment when I was single or in a long-distance relationship was no longer tolerable once I was married and cohabitating.
Another thing that isn't being considered is the age of the people doing this work. With the benefit(?) of looking back at my twenties from middle age, something that many of my friends and I have talked about was that we all went through radical changes in our self-perception and aspirations around the age of 27. I was beginning to feel unfulfilled in the video games industry, another friend of mine was already working on a major political campaign which had been a goal of his since high school, another friend had gotten a few years into his dream engineering gig, and we realized that the goals of our school years were not what could fulfill us going forward. 27 isn't even hitting hag age. Some of the talents have got to be thinking "is this what I'm sticking with?" and the feeling of emotional debt towards the fanbase is what's keeping a non-trivial number of talents from going indie or quitting.
To a degree, these vtubers at the extreme end of popularity and income have an advantage that many people don't have for being able to step away from these jobs if they no longer feel invested: they don't have children and mortgage payments that are predicated on the six figures that most Hololive talents are clearly bringing in. If you have kids, you'll stay shackled to the grinding corpo job that requires overtime because you're able to provide for them. If you have a mortgage, you will engage in the yuppie Nuremburg defense and do whatever it takes to stay in that house.
I'm not suggesting that there are no problems at Cover. Even the best workplaces have their problems and a company that has been through the growth that Cover has is certain to have problem actors in addition to nonsensical policies, meeting requirements, or general East Asian Corporate fuckery. But a lot of that stuff can be framed so as to be tolerable. It's when those feelings of meaning and satisfaction evaporate that they become intolerable. Old man brain-vomit, for what it's worth.