I guess this must be it. IRC could in theory do everything Slack does but setting up a server is a pain and shiny clients are rare...
I wonder about the rest of the competition, for a while everybody was using hipchat, I never used it myself, I wonder why nobody talks about it anymore.
There is competition out there, they just haven't made enough noise yet. See Fleep for example: https://fleep.io/, Rocket Chat etc.
One of the reasons is the lack of competition, e.g., we use Microsoft Teams instead, but its also an electron app with the same problems (and takes ages to start).