Imagine, there's an app with Slack's functionality that already has such interop with email as you've built here :) Fleep: https://fleep.io/

Anyone who does not want the group chat experience can be included in conversations via email.

Currently, that does mean getting all messages via email, and not @mentions (nice idea, worth considering for Fleep's product team). But idea is similar:

Full disclosure: I do work at marketing in Fleep :)