1) The UI/interaction/UX can (and obviously will) be replicated, which has been slack's biggest value proposition.
2) There's no "stickiness" for companies. None of the data in chat is really a "system of record" and the switching costs are minimal. 3rd party bots/integrations are the only thing that really make it sticky for companies.
3) The IP isn't really all that interesting. Chat based systems have been around since day 1 of the TCP/IP protocol and it's design patterns are pretty well known. In other words, the tech can be replicated.
4) It's not solving a core technology problem for most business without introducing additional problematic externalities. http://www.businessinsider.com/i-used-to-be-obsessed-with-sl...