[amsha]

Email federation is more than just the addressing system. Even with a consistent address, your host still needs to be able to send and receive SMTP messages to participate in the ecosystem.

Address lock-in is real but doesn't fully negate the benefits of federation. Even if you don't control your address's domain, you can switch providers without losing the ability to send and receive email. There is extra cost in pointing your bank account to your new address, but it is possible. If your bank communicated to you via a non-federated protocol like WhatsApp, switching would be impossible.