It sounds like they didn't do any research into what Slack's limitations are. Also, frankly, if I were Slack I would not invest in supporting this use case at all. Massive free chat rooms are not a profitable space to be in.
It depends on the community. For this one, it's a bunch of folks learning to code and getting coding jobs; I can imagine many of those ending up working at tech companies who might be great Slack customers, and evangelising because they had such a great experience using it - so the investment of supporting a 'free' chat room could pay off over time.
The OP sounds very entitled: "We'd endorsed Slack to thousands of people on our Twitch.tv streams, and even mentioned it in interviews with the media."
Of course Slack must provide them with free chat rooms in perpetuity, they've even been mentioned in interviews!
It's probably better to be rid of "customers" like this sooner rather than later.