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Multiple JP holos mentioned that they have a minimum stream quota in their first year contract. I assume it's in there so they at least break even on their investment. They have to pay for your model, manager, marketing, including you in stuff like holofes, merch that includes all of hololive etc.
I think the difference between her and Ayame is that Sana never broke even on the investment the did. Council's artists were all high profile. They probably weren't cheap. They probably just didn't renew her contract.
Also, Ayame actually streamed in the first two years. I don't like her because she's clearly lazy these days but nobody can deny that there probably wouldn't fucking be a HoloEN if she had sat on her ass like Sana did during her debut days.
As for the Nepotism thing. You really think it's a coincidence that Ina's friend got hired out of thousands of applicants? Sana was never a good streamer while everyone else in EN either had streaming experience or a skill that made them stand out.