Those numbers are not true for even half of the world. Also, I would want everybody in my org to be part of the same communication channel including every non tech folk.

You will definitely find folks for whom the $100/mo/user is not a big deal and Slack does have that plan with needed features. However if I am bootstrapping with a business with majority of non tech part time employees, its not a quick decision to commit to $8/mo/user.

For comparison, Google Apps provide their complete suite at $5/mo/user. Desk.com provides interfacing with customer starting at $3/mo/agent. With the value they provide I don't think twice before committing to it.

(btw, I could only see their cheapest plan at $8/mo/user paid monthly, where is $5/mo coming from?)