The company I work for uses Slack, and luckily there is an IRC bridge to connect to so I can keep my workflow in Emacs rather than having to use a clunky web interface, but there are many special Slack things that they don't render correctly in plain text form so I feel like a second-class citizen. If this is the future of text-based communication, I don't want to participate.
I like IRC. I like that things aren't centrally logged by default. I like the bots that I've been interacting with for many years (that I find to be better than whatever Slack bot I've come across, by the way.) I like Freenode as an IRC network. I like the variety of clients available (I prefer ERC, a client written in Emacs Lisp.) Could we use some sexy web clients to get less technical people on board? Sure! Should we use a proprietary, centralized, surveillance system until someone else fixes IRC issues for us? No way! We must reject Slack because it isn't a tool for the people, but a tool for profit.