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.