The problem with the product is that it really exists to keep big companies on the O365 E3+ plans.

Microsoft's big focus now is to retain subscription revenue. Skype and OneDrive are shit and MS is trying to bulk up the ecosystem around these products. If you work for a big company, you're not going to get approval to POC Slack, Dropbox, Webex, etc as long as you're stuck with the Microsoft all you can eat subscription.

Slack is a killer threat because it displaces email. Exchange has always been the glue that holds the biggest Microsoft cash cow together.