I don't see any compelling reason to adopt Slack. It does nothing that I care about that IRC doesn't do. If IRC completely implodes, I would run my own IRC server.
I personally love Riot and Matrix as a "modern IRC".
It does require having your own server to host it on though.
That's nice for you personally, but how does running an inhouse bespoke IRC server have anything to do with whatever line of business your company is in?