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.