I mean, you can do that already, you use your own domain name and can then change email providers at will, in theory.

But maybe you logged in to your domain registrar through google oauth. If your google account is locked you can't now get into your domain's settings to change your MX records.

The real problem isn't the email address itself, it's all the access that google owns on your behalf. Lose access to Google, lose access to everything.