I wasn't intended to compare IRC to Slack but since you've started us down this rabbit hole: I'd take IRC over Slack any day. Sure Slack is more "modern" and it can - in theory - do anything IRC can do. However IRC is more easily extendible and in a more versatile way. To be honest there's nothing Slack does that you couldn't also do in IRC - albeit some features might require a more modern IRC client than the terminal ones us geeks often harp on about (disclaimer: I've written tools for both IRC and Slack).
The problem with IRC faces isn't a technological one - it's just a mixture of NIH (not invented here) syndrome and capitalists wanting to make money from reinventing public domain tools as proprietary solutions with expensive subscriptions. Not that I have a problem with people wanting to make money - quite the opposite in fact - but I do take issue with people who say "this can't be done in X which is why Y exists" when the real reason is financially motivated rather than any technological shortcoming.