Can you run a Slack competitor on top of this?

I don't think that would be a good use case.

That's the thing... If you limit the use case enough everything is safe and portable.

Oh ya, I'm not saying it's the best software architecture for most apps. It only works for a small subset of apps.

And it's good to have this in mind. Sometimes people go to the solution with the most batteries included and import lots of potential vulnerabilities without any real benefit. KISS should be the rule.

On the other hand it's hard to know if you are eventually going to need more batteries in the future and you risk having to migrate your solution stack. It's kinda the same with no-code platforms, where it's very easy to do basic CRUD but once you need to do anything slightly more complex you need to reimplement everything in a real language.