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.
I would argue that email is not suitable for asynchronous long-form threadful discussions. The limitation that email has is that if you're not part of that conversation from the beginning, you'll have to piece it together from previous quoted material.
One email like protocol that properly handles this is NNTP.
True regarding the late-comer aspect, although it is less of an issue when using mailing lists with an archive. In the past, when lacking an archive I also just asked another participant to send me the earlier discussion in mbox format, which was easily accomplished with the unix MUAs of the time.
Regarding the actual modes of discussion I was thinking of though, usenet and email are mostly the same.
> 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).
