The barrier to entry to compete with slack is fairly low. And Discord can just decide to start focusing on enterprises. Microsoft is just one company with the leading office suite that happens to be really kludgy. Slack has its pluses and minuses. So just because slack works great and is fun to use, and has lots of companies giving them money, doesn't mean they are worth their huge valuation. They aren't getting enough money, and another hipchat like competitor can come along and do even better. The best thing slack has is they are the default safe choice.

I say this as a slack stock holder who lost money, so I clearly know what I'm talking about ;-)