No, it’s when you decline to federate content that some users don’t want (and many do).
Yeah, that’s definitely citation needed.
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).