I really would disagree. IRC didn't have a lot going on, you had channels and PMs and that was kind of it. It was very easy to learn how to use because there was very little to learn.
Not to mention, in most clients, it took significantly less than ten seconds for the text to appear on the screen after you started typing.
There is a lot more to IRC than just channels and PMs - it's just that not many people have purpose to find out about anything else because channels and PMs work so damn well for their purpose.... communication.
While I agree and personally prefer IRC, people using slack use far more features than just channels and PMs. That's because slack is more discoverable and the features are generally conceptually simpler and more obvious for the average user than complicated IRC features. That's the UX I was talking about above - it allows more users to use more features and feel more comfortable that they know what is going on. The same really can't be said for IRC - most average users will request the help of an 'IRC master' if they need something they don't know how to do. That's probably not a good sign of accessible UX.
Not to mention, in most clients, it took significantly less than ten seconds for the text to appear on the screen after you started typing.