Great, let's legislate that you can switch providers but you have to be able to keep your email address, like we did with phones.
I'm not aware of any mail providers that require DNSSEC. Were you thinking of DKIM? That's just 1 more TXT record (to publish the public key used to verify the signature), and some mail signing software if your mail server doesn't have that feature built-in (which is freely available).
...Or are they, except not in cash? :) Jokes aside, that's a fair observation, but then one should be able to "transfer" their address by paying a one-time fee, rather than getting a GSuite subscription.
The nice thing about a law is we can figure out how to do it after, not before. :)
It wouldn't be difficult! There are 7.6 billion people on the planet, an average email address is probably 25 characters. If every email address is forwarded, that's ~380GB of forwarding data (from address + to address) - and keep in mind that's the stupidest implementation and the worst case possible. I'd like to think that someone who offers a public email service can reserve 380GB of SSD for a forwarding table without going out of business.
Practically, I'd expect vendors to quickly agree on a "301 permanently moved" scheme. So if a Yahoo user is sending an email to a GMail user who moved to a private mail server, Yahoo wouldn't even bother pinging GMail (after the first time) because they'd know that address was moved.