The only problem with IRC and many of the open protocols (XMPP too) is that there is no "one true IRC": so while you can get all the same features as Slack, you've got to choose a particular server, a particular set of extensions (I am sure ChanServ/NickServ stuff eventually got "standardised" like DCC stuff did, but remember that DCC file sharing was direct-IP comms between clients), and on a specific client with the same set of features.

Slack and other tools are obviously inspired by IRC, but they do resolve a bunch of issues that IRC never got properly solved (or maybe did, but deploying it was too complex or it came too late).