Exactly. There's no replacing email with something that's closed membership / invite-only etc. The whole point of e-mail is its your address where someone can i

Exactly. There's no replacing email with something that's closed membership / invite-only etc. The whole point of e-mail is its your address where someone can initiate a contact, like phone number. You can choose to filter it etc. And you can choose to get attested emails which you can trust more etc. But fundamental nature is it is open for anyone to contact you. If you lose that, and build a custom new protocol, you are just building another closed wall messaging application.