Rosemi's a quiet ringer, basically. Aside from recently getting noticed by Harada she doesn't make a ton of waves so much as she's a rock solid foundation. She's the super-nice, likable girl who don't mean no harm, just about everybody that knows of her likes her to some extent, she gets on well with just about everyone, etc. etc. While Selen was a visible, public dynamo, Rosemi is more of a metaphorical heart. As long as it's beating, the body's still alive, even if just barely.
She's essentially in a similar (but not identical) position that Fubuki has in Hololive; if she leaves, that's the sign it's over. Although we know Niji's black as fuck, if it gets to the point they've driven off Rosemi of all people--someone that's harmless, likable, and just wants to game and get along with people--that's the final horseman riding on through.