But clearly no one is saying that email is too hard to use and we should just use $something_else (or are they?).
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.
They are moving the goalposts because there are several and ultimately very many reasons why IRC won't work, they just didn't bother to think of all the reasons and list them at once.
Ultimately there is only one reason that matters: The person in charge of deciding what communication channel to use likes Slack/Teams/IRC/whatever.
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>
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?