I think that is currently already in the process of happening. The reactions to the announcement from Beehaw users can generally be split into ~~2~~ 3 categories:

1. Support, we should keep our communities insular

2. Neutral, I support whatever the admins decide

3. Opposition, I'm leaving to an instance that isn't insular

EDIT: Added neutral, because on reflection there were quite a few posts like that

This means they are already self-selecting for insularity, which means the resulting userbase is very likely to want this "temporary" solution to become a permanent one.

This isn't necessarily an issue: if the userbase is happy with their insular nature and are comfortable with it, and it's clearly signposted on the sign-up, then after some network healing where we build communities separate from Beehaw everybody gets what they want