Slack targets the same market that made Microsoft big: the corporate sector. For big companies, even recurring licensing costs for Slack are trivial, and it gives the managers of the bigger companies a feeling of importance to be able to negotiate big bulk discounts. In this corporate sector market, all you need to do to get rich is to be the established player, which Slack very much seems to become.

As for the easily being replaced part, Slack will just buy up any competitor while its small. Just like how Facebook did it with Instagram and Whatsapp. Facebook is deemed uncool by the younger generation but not Instagram and Whatsapp.