Slack to most end users is the client, though. As well as the admin, user management UI, scalability, webhooks, etc, absolutely none of which is handled by "you can chat with other users in Linux if you all log into the box" which every *nix has had since before most readers here were born.
Indeed ! The question is whether all that is worth so many engineer-hours. I'm not advocating for all Slack channels to be replaced by suc, that would be silly. I'm just pointing out how costly it all is. There's got to be a middle ground that does not require hundreds of thousands of lines of code.
