I don't see how you get that. Slack does tons of things IRC doesn't. IRC is not a valid replacement for the entirety of slack. To choose IRC means deciding that you don't need or want the rest of slack's features.
If this is just going to be a game of "let's list every feature Slack has bolted on to their chatroom" then I don't care to play.
I don't get the stance that this is "hype" or a "bandwagon" at all.