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Pic related.
If you look closely, you'll see the top 30 Niji talents don't compare to the top 30 holo talents. But Nijisanji has more viewing time than Hololive, specifically because they have hundreds of smaller talents. They add up.
I think this way of doing things started, becaust Nijisanji made a phone-app for convenient streaming on the go. The idea was, after developing this app, just spread it to as many people as possible, let them do their own streams, and take a portion of the revenue. In that sense, the more streamers you have using your app, the better. 100? 200? 1000? The more the better.
Originally the JP branch released "waves" of talents. But soon they gave up on that, and started just debuting vtubers all the time. The EN branch was constructed to more closely follow the Hololive model. For example, in the main branch, talents are often openly in relationships, or openly married. But in the EN branch there is a big taboo about mentioning relationships (although not a strict rule). And the EN branch releases waves of talents, kind of like Hololive's "generations", and has established a tradition of giving each wave their own original song at debut.
But it's kind of a vicious system when you think about it.
-- Debut brings in lots of new viewers, but also stretches the old fanbase thin among more people.
-- Once these new viewers get interested in the company, they cling to the most talented vtubers, surging their numbers even higher.
-- Meanwhile the new debuts stretch thin the viewers of the poorly-performing talents, forcing many of them into graduation.
It's basically a system that kills the weak, and rewards the strong. Keep debuting more people, driving your weak vtubers into graduation, while the debut highs surge the numbers of the successful people even higher than before.