Speaking as an open source advocate and a believer in open systems: IRC and Slack are not interchangeable. Not at all.
IRC is arcane. You need to have a special client, or a really ugly web client, to use it - all of which kind of look like something out of WarGames circa 1983. You need to memorize slash commands. You need to remember which federation of servers to join - and even then, networks sometimes split inexplicably, leaving people isolated from each other while sitting on the same channel.
Once you’ve made it through all these technical hoops, more often than not you find yourself in an unfriendly community with no auditing or oversight. Not to mention that running an IRC bot is an unfriendly and arcane process in itself, making the kinds of integrations that people run all the time incredibly hard to configure and run for yourself if you’re not a system administrator.
Slack is definitely inspired by IRC, but it sands off the rough edges and uses that mechanism to create something much friendlier for real people. I would never try and run IRC inside a company again, but I’m not sure I’d live without Slack.