Email is dead. Unfortunately any replacement for it would be owned by Meta or MS or whatever.
The assumption of a single shared medium is dead.
When you talk about a single entity to single entity connection it can be any protocol. Email, Matrix, WhatsApp, SMS... Discord... Instagram. All of them work especially if you whitelist the other party.
What doesn't work any more is assuming that everyone is using the same thing. Back when email was invented it was the only computer-based communication protocol - I don't remember whether it came before or after USENET - I'm assuming before. If you wanted to send a message to someone you sent it to their email address. Now things aren't so clear.