Quinn knows how to disguise his dark deeds. Who knows about prostitution and drugs? Only her inner circle and clients. The same thing will happen to the one who starts talking as to Mona. There are no clues for Sage that would indicate that Quinn is involved in illegal activities. So far, the only hint for Sage is that Quinn is a bad leader and causes trouble.
That looks like typical Reddit/twitter user mind logic here. If the person does one little bad thing, that person is bad to you by default and should be canceled. That's not how it works. People make mistakes and Sage was in a really shitty place at that point because of her relationship with Chad.
Sage showed concern for MC when he got beat up by the jocks, she supported him after the J/M situation, she showed empathy for his dead mother, we saw her warm side when she was sick, Sage took it upon herself to help Maya, a person she barely knew... just a few examples out of many, but Sage is bad because she didn't give up the guitar right away.
She acts as a leader should - sends her VP to fix her own fuckups, takes over the situation with Maya and tries to contact Mona. She solves problems with DIKs and Jocks when Quinn does nothing with one conversation, implementing a policy to solve problems and here we saw Sage in all her glory. Then she removes Quinn from management and essentially takes Lily as an assistant, concluding that Quinn is a bad VP. Why did Sage not fulfill her obligations? Game shows us this - Sage is emotional and sensitive, she goes through drama with Chad (this is a separate topic about how involved she can be) and then the flu.
You are free to like/dislike anyone, just your line to argument "Sage bad" didn't work.
~
And who's your best girl?