I think that you just described XMPP and Matrix, which are both standards.

Comparing XMPP/Matrix to Slack is like comparing Telnet to Chrome. Yes, they both display text, they're both interactive. But the latter has about 5,000x more features, which is why we all use it.

Yet with Slack, it doesn't use a standard. Could you build an app like Slack that includes XMPP/Matrix along with a whole lot of other stuff? Sure. But without the whole kitchen sink, you still don't have a standard other apps will follow. You have a proprietary app plus XMPP. Other apps won't be compatible with it. Which is the case with Slack's competitors.

Think of a web browser. It's larger than a kernel. It's probably the biggest, fattest, meatiest, most feature-rich application in the world. (And it should be, because it's a freakin' application platform at this point.) But it all runs on.... standards! Every part of it. I'm saying, do that, but for the massively feature-rich, complex, large, almost unwieldy, but insanely productive, communications platform that is Slack.

I get that a lot of people don't really understand what the big deal about Slack is. A lot of people thought the same thing about web browsers back in the day. But once they started using them a lot, they got it. It's not just a document viewer, just like Slack isn't just chat.