Earlier quoted context omitted.
Word. I don't love Slack, but IRC is awful, and an open protocol replacement for Slack would have to make easier many of the features that Slack has and IRC lacks.
What features does IRC lack? The messaging, chat rooms or PMs, private servers, file sharing, etc has existed forever. You can also build any sort of tool for integration with it. I guess it lacks persistence and access to history? But there are surely ways to solve that.
persistence, history, search, rich formatting, image previews, synchronization between devices, usable mobile clients, group/here/channel mentions, channel directory/search, channel descriptions, file upload/sharing (XDCC is not this), multi-line/expandable messages (eg, paste code or logs without flooding the channel), user status (away/DND).
Some of this can be approximated with bots, of course, but there's a lot of extra work then required to make that work (namely, setting up, writing and maintaining the bot). Some of this can be done client-side, but then you have inconsistent experience.. you don't know if the other user is going to get the image preview or if their client understands markdown or some other formatting, or is going to interpret and display your message using some other markup format. Some of this you can also work around with one of those persistent IRC connection proxies, provided each user gets one and that is also maintained by somebody.
Eventually someone packages this stuff all together for simple deployment.. and basically creates a bad clone of Slack, but with 10x maintenance requirement and so many more things that can break.