Slack has raised insane amounts of money at really good terms just because they can. The funding world is insane right now and they are taking advantage.
They can easily do what Facebook or Oracle did and just buy up any competitions with their huge warchest.
You can't always buy out your competitors. Facebook itself is an example of this, having turned down numerous acquisition offers (including one from its competitor MySpace, and then one from the company that bought MySpace):
They can easily do what Facebook or Oracle did and just buy up any competitions with their huge warchest.