Email is not a chat system, and chat systems are unsuitable for asynchronous long-form threadful discussions. There is some overlap, but combined they form a spectrum of communication modes so wide that it can‘t be covered by a single UI.
One email like protocol that properly handles this is NNTP.
Protocol: https://github.com/networkimprov/mnm/blob/master/Protocol.md
Why TMTP? https://mnmnotmail.org/rationale.html
Follow: https://twitter.com/mnmnotmail
Regarding the actual modes of discussion I was thinking of though, usenet and email are mostly the same.
For the most part, they are and many readers support both protocols (or at least they did in the past). The nice thing about NNTP is that it doesn't require maintaining a separate archive or having someone send you an mbox file to import. Just subscribing to the appropriate groups was sufficient (depending on the article retention policy).