Slack is (or should not be) mission critical. If Slack shuts down, you can still communicate with your team through email, SMS, or any other alternative. Your product will remain in operation even if Slack dies.
Slack is not mission critical, you mean.
Oh noes, slack is down we can't deploy :(
Slack changes a decent amount of money for access to their platform.
Slack is gaining a lot of momentum in the enterprise space as a collaboration platform. Considering that they are basically a supercharged IRC, I'd say that's pretty nifty. I don't see them shutting down any time soon.