We use Slack in a company of ~30, and while all the devs could easily use IRC, most of the rest of the company would struggle with it, and certainly wouldn't be able to communicate in the wide variety of ways (uploaded files, images/gifs, videos, code snippets, etc) that they do now. Slack is great at making all those rich media types 'just work', and while it was possible with plugins to some IRC clients, the UX was prohibitively bad in my experience.