I think half of the "irc > slack" crowd are trolling or willfully ignorant, either one as a form of technical elitism/hipsterism. Slack is a nice UI, has a nice app on nearly all modern mobile platforms, has drag-n-drop uploads, 10,000 message history for free. Even if none of it is perfect, it has all by default. I have been informed that you can do a bunch of really complicated stuff to mostly match these features in IRC while also taking the responsibility of maintaining it. I've even been told the lack of receiving messages when returning from offline is itself a feature (for business use, "lost messages" this makes it dead on arrival). But when I show my less technical boss or startup co-founder "deploy new slack team quickly in 2 minutes" vs "IRC can be selfhosted, but maybe can be brought up to slack's feature parity in some indeterminate amount of labor hours", I think the choice is obvious.