Reality check: Most people don't use email programs anymore.
Also how do you get IRC to sync all conversation data, history, between your several desktops and phones, how do you send files, make calls, and thread conversations?
And you are starting to move goalposts here.. first it was uptime, then it was operations and now it's features...
And what about those web based IRC solutions? They are even easier to use than slack, have combined history, file sharing, etc.
Add to that the SaaS propaganda that hosting literally anything yourself is just too hard (it really isn't). Or this notion people are just too stupid to deal with anything more than the simplest possible web interface - Really? what do those people even do? Stare at Notepad all day? Of course not. They stare at various complicated software packages ranging from CAD, $spreadsheet abominations, SAP to various Adobe software packages. Sprinkle in a bit of hype for the latest new thing and presto.. </rant>
Guess you are not in enterprise.
Also there are plenty of modern web clients for IRC, such as https://thelounge.chat/ or https://kiwiirc.com/ (which is supposed to work on mobile too).