Let's say that Kirsche had a stream scheduled with Carl in a week's time.
Now, let's say there are a few types of people that would see this upcoming stream announcement:
1) Kirsche's audience, or vtubing fans that are interested in Carl or his content. They''re tuning in. Kirsche might even get some subs from people who don't normally watch her.
2) Carl's audience. Some of Carl's audience would be completely against vtubers for whatever reason and will just skip the content, and that's fine. But some of them will tune in for a Carl stream, even if they think vtubers are a bit weird or don't really care about them. They're a bit apprehensive, which is fair enough, but they'll give the stream and Kirsche a chance at least. And after the stream, some of Carl's audience will have enjoyed it enough to keep track of Kirsche, maybe sub to her and try out some streams.
I'm all for gatekeeping undesirables out of vtubing, but saying Kirsche shouldn't stream with anyone that isn't a vtuber is a bit over-the-top.