Slack's bait and switch
With all the (appropriate) smugness of "I told you so" surrounding this issue, I think that many people's attitude around sticking to IRC and mail is a reason why something like slack could even take off so quickly.I regularly speak to people like that, who just plain refuse (are unable?) to even see the difference between a chat like slack (or telegram, or mattermost, or or or...) where I can post images/videos inline, use proper markup etc, and a combination of IRC and email.
"But you can just send images by mail!" they shout. Yes, you can. But the user experience will be a different one. And it doesn't even matter that I personally prefer the slack-like UX.
Many other people seem to prefer it too, that's what matters.
For anyone who's only mildly technical, setting up IRC is only a small hurdle, but it's one of many.IMHO, if you want people to use anything else but slack, sticking your head in the sand and screaming "you can do all of that in IRC" won't get you anywhere and is equivalent to complaining about the very nature of humanity. It might feel good to scream out your weltschmerz, but it won't change anything.