But that's true of... any email provider, though? You'd lose access to your ISP email account if you switch ISPs, etc.

Federated email just generally sucks, insofar as email addresses are expected to stick around indefinitely, yet are almost-always associated with some service provider you might eventually no longer be a customer of (for whatever reason) and with "owning your own email providing" being nigh-impossible for deliverability reasons.

Also, people's emails to you wouldn't go into a black hole; they'd start bouncing. Which would mean that those people would realize that they should be reaching out to you some other way.

(In fact, properly-coded 2FA systems should recognize an SMTP bounce and use it as a signal to temporarily reduce the level of authentication required to reset 2FA. Things like this are why it's so hard to get 2FA right.)