I'm probably getting old, but I've been using it for about 3-4 years now (because work) and it seems just like a polished IRC client tied to a proprietary service to me.
It's not just a dancey, polished IRC. It _is_ a dancey, polished IRC that invites people from all backgrounds to use it and enjoy it. As a communications glue for a company, I find it to be very valuable in talking not just with other developers, but other departments.
This is precisely the point. Most companies just needed something that worked. It doesn't really matter if it's proprietary, if it gets the work done.
Paying a little money to slack is much more convenient that getting you entire company to setup and use IRC.
Out side the HN bubble, people choose convenience over open and free.
The only problem is that it's not IRC, it's another closed source data gathering operation.