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/