Slack definitely started from the software niche, but is far from the only player in the field.
Others mentioned Mattermost, Discord, but really, there are so many viable options out there. See this list of 25+ alternatives for example: https://fleep.io/blog/best-slack-alternatives/
From cloud-based to self-hosted, open-source and commercial... As long as they integrate with the tools devs use, there will be devs who use it.
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 :)