We would basically need a means to migrate an entire community to a new instance, while simultaneously updating everybody’s subscriptions to reflect the new home of the community.

That would be nice. As a regular user, when lemmy.world does something you dislike, like block piracy communities or something, you can simply create a new account and, until something official exists, use LASIM to migrate stuff over. I didn't think about communities though, if you run the biggest community for some topic what do you do. Create another one, link to it from the first one and hope for the best?