Slack and Dropbox have pretty different business models from my vantage point. Slack entered the market as an enterprise product vs. consumer. Slack was built for collaboration from day 1, so there is some amount of lock in (change management in an organization is tough).

I look at Slack as being more like a hybrid of the best parts of Box + Dropbox... They have the good parts of Box's business model and the UX quality of Dropbox. And fortunately for them, they aren't in a market where the price is driven largely by the cost of the commodity hardware underneath them.