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.
