> who just plain refuse (are unable?) to even see the difference between a chat like slack (or telegram, or mattermost, or or or...)

What if they do see the difference and don't see the value in it? This is worth considering.

> where I can post images/videos inline, use proper markup etc

I don't want someone else putting inline videos or markup into my feed. In IRC people post links, most of which I happily ignore at very low cost because they're not important to anything.