If I’m running lemmy.world, I wouldn’t unsubscribe my people. I’d wait for that instance to move to a new domain and just find/replace in the database.
Not every instance needs to migrate fmhy. Some can just leave that stuff broken. If the biggest half dozen instances migrate manually, fmhy would be able to keep most of their subscribers.
I do wonder how often instances will keep looking for fmhy without intervention. Seems like tooling to migrate or discontinue an instance wouldn’t be too difficult to build. At least it wouldn’t if they didn’t have a million other things on their plate.
We could use a few less third party clients and more work on Lemmy itself. Unless you’re going to bring over your userbase like RiF and Apollo can.