And this is why, back in 2007, I registered my own domain, and signed up for then-free Google Apps for Your Domain (then GSuite, Google Workspaces, whatever they're calling it now). Earlier this year I moved my email to Fastmail, and I can move it elsewhere if I want to, with zero disruption or downtime.

I really wish email providers would make custom domains either the default, or a very obvious option when signing up. Google is already a domain registrar, and other providers could partner with one. Granted, this option would not be free (though Google could probably swing making it free; they just wouldn't let you use the domain for anything else unless you start paying for the registration), so that would reduce its desirability for most people, unfortunately.

(On the downside, I wish I could convert my GSuite account to a regular Google Account, because GSuite accounts are occasionally crippled in random ways, and now I'm not even using the email part of it anymore. But that's a separate complaint.)