I see this IRC argument come up quite a bit every time Slack or like thereof are mentioned, especially on HN because - you know hackers, quite a few of us used IRC and still have a bit of nostalgia about it. However, please consider this as a thought experiment - how many of your family members, co-workers (especially people from non tech/non STEM backgrounds) were able to successfully use IRC or you used IRC to communicate with. When we had IRC server at my current job before we switched to Slack, it was an insular club of engineers with a high bar of entry - you need to know how to setup clients and create alts if you wanted to use on multiple devices and know decent amount about IRC to use IRC. Now contrast that with Slack, it is used across Sojern by 100% of our team - commercial, marketing, product, c-staff, and engineering alike. UX is designed to get out of your way and make it easy to communicate rather than act as a "filter" or "bar" or "must be this haxx0r to use", in addition my family has a slack workspace we use for various things and that includes - again the least technical member of my family - my mom, who is a Pharmacist but not the most tech savvy - she has zero issues using Slack.
It is easy to dismiss something as "just a chat product" but then again, it is incredibly hard to make a product that is so easy to use that everyone can use. Reminds me of that comment about Dropbox from years ago https://news.ycombinator.com/item?id=9224