I mean if Slack didn't end up ruining their API in the name of greater development flexibility I'm not sure I could even comprehend it.

But that's the thing, I think Slack is a fundamentally inappropriate tool for FOSS work or public communications. Anyone who stakes their long-term community on a single company's proprietary product is asking to be hurt down the road. But I don't think IRC is the right alternative because the UX is so newcomer unfriendly and requires so many disparate 3rd party services to be useful. I would much rather see communities standardize on something like Riot or Mattermost with OpenID.

Simultaneously, I think that Slack (and its contemporaries) are a breath of fresh air for internal team/office chat. Sure it's expensive, and like all corporate-y things you have to keep a little distance so you can migrate when they get shitty, but we can at least acknowledge that it's a damn slick product.