How is ActivityPub instance migration worse than switching email providers?
If you're using someone else's email domain (like, eg. gmail.com), you cannot migrate to another provider. If you use your own domain, you can change who is hosting it.
Like email, there are plenty of Mastodon hosting companies. You can use your own domain, and migrate between them at will. If you use eg. mastodon.social, you can even migrate in most circumstances (but not if the instance vanishes).
OTOH, eg. (not an advertisement, just the first search result) masto.host offers managed Mastodon hosting for $6/month, which is on par with eg. Fastmail or Proton's base non-free plan.