Hard to really fault him since he ran into an undisclosed max user limit, no?
Unbelievable that a service like this would change the key part of their platform without even checking the pricing page to see what free plan restrictions were...
EDIT: I stopped reading after the first paragraph, looks like there was a user limit issue too :)
Slack's pricing model and the ad copy of their website makes it obvious to me that it's a communication tool intended for small teams, not a chat room for thousands of users.
My university uses Slack, and the "main" rooms see a few posts a day, but there are tons of small groupchats and DMs.