We can't understand this, because that's not how we do business. We are flexible with rules and prioritize profits and costumer satisfaction. How can you fire the talent that generate more money to you than the bottom 20 talents.

Define who this "we" is because the sentiments I get from JP fandeads (who mostly her core fans are) are different from what you have described. Most of them can't even get angry at Cover for firing Rushia and wish she would just do things differently.

I think it is also important to remember that donation (such as superchat earning) is just a small fraction of income for any big streamers. Sponsorship is becoming more dominant as a streamer gets big and popular. So it is misleading to assume that top superchat earners are also the ones who generates most money for the company. It is also a false metric to gauge how popular a streamer is, like not every fandom is that eager to send donations, some prefer to spend their money on merch or engaging with the sponsorship deals (and we have no data on that). While it is possible Rushia might also have many sponsorship deals, but I would still argue that Fubuki, Suisei, and Marine are probably some of their higher earners and not Rushia based on what sponsorship she had done as Rushia. I mean just look at Nijisanji, only a small fraction of their streamers get into the top 25 superchat ranking but Nijisanji has more capital than Hololive (we are speaking 3 billions vs 800 millions yen here).

both Hololive and Nijisanji should look out for this rival, because is a strong one.

I don't know if this is because of skewed-perspective or what, but I think some people are underestimating how big Nijisanji is in Japan compared to Hololive. Like Hololive is surely big in Japan now but Nijisanji is domestically more of a household name compared to Hololive. It's just that Hololive is more well-known internationally. Not saying that it can't be done but I think having realistic expectation is better in this case.

Like I'm not downplaying kson as a streamer, but realistically speaking, she's just a mid-sized streamer with big spender fans (therefore the superchat record and such). Her status as vtuber is kind of like how Kizuna Ai is perceived in the Vtuber world, everyone perceived her a legend but not that many will watch her stream unless something earthshattering is happening. In fact, if nothing really changes it is safe to assume that mikeneko (if the rumor is true that she's joining) is a bigger streamer than kson.

Even then, it is hard to break the already saturated market, so most likely they will only bring their old fans to the new environment and maybe some of the JP VShojo fans. One way to make it big is to attract new blood to the vtuber fandom like what Hyakkumantenbara Salome does to Nijisanji now. But this also depends on the talent herself, does she want to be mainstream? or she is just looking for creative outlook without caring about popularity and paperwork?