You can't call keeping merely the contacts without emails themselves "migration of email provider". It makes no sense.
Also, you said "follower[s] don't have to do anything", but somehow you (the followee) on the other hand needs to actively move? What if my follower is on this instance too and they don't actively move? Shouldn't their account disappear (and you lost your follower)? I genuinely don't understand how it would work other than everyone has to manually move together.
That's mistaking the map for the terrain. It's a networked system for leaving and retrieving messages. It's not a group of friends. The way you expect to migrate a messaging system is by moving the messages.
I'm not the person who said that but I can take a swing at it:
If the person does not migrate off of the instance, they'll lose the account and yes, you'll lose a follower. But if they do migrate, both of you keep the connection.
It doesn't move your toots, but followers don't have to do anything to still follow you on your new account.