Lemmy

In this case you have to be posted AS A crosspost to take effect, and any one of the cross posted community can just delete the post, or presumably uncrosspost it.

The problem usually is that, nobody bother interacting with small communities and aggregate around the "one big community" for that topic.

Small community who would want to remain insular have lots of ways of disappearing further if they want to, but that's never the actual problem of small communities. It is always easier to have less reach and become less relevant than the opposite which better crossposts enable.