Through legislation? I suppose you could legislate smtp redirection / forwarding. It won't be seemless though. Google for example would never permit me to host their domain, so it would have to be a redirection or something like that which you can technically do today minus the risk of them killing the forwarding and/or account. I suppose the legislation would have to require the forwarding remain in place for a set period of time.

The technical solution already exists. Get your own domains, point them to a provider of your choice, set up forwarding on google/yahoo to your new domains. Go through all your services and change your email to use the domains you control. If the provider becomes compromised, toxic, etc... you can change DNS to a new provider. Use imap-s to back up your folders so that the email provider can't leak your data.

Yah, the tech behind this already exists. Just needs to be packaged together.

Legislation will go together to require a large free email provider to also provide tools to set up a forwarding service and operate it for a period of no less than 20 years, as a starting point.

This idea assumes that email is an irreplaceable source of personal or organizational identity.

I believe that assumption is a misapplication of email. Universal identity is a complex issue, and is not addressed well by email addresses. Portability is one example.

Also the cybercrime crisis, which is substantially enabled by email phishing, may well drive email off the public Internet. We no longer need an Internet messaging system which makes you accessible to everyone else on the net, without consent or limits.

For a next-gen take on Internet messaging, see the mnm project, which proposes and implements TMTP. That system supports separate site-specific identities for every user (and multiple aliases per identity), which is a better match to the real world.

https://mnmnotmail.org/

I think the underlying question is "should access to email be regulated as part of the public carriage, public utility function"

When google shuts down access to an email box, If it continues to 200 OK accept SMTP, its potentially accepting contracts, warrants, government communications...

Email is portable. If you use your own domain, you can switch providers without changing your address.

Most providers offer email export and import. So you can also move emails between providers.