Yeah, I don't want to be a downer, but this is pretty much the exact definition of Embrace, Extend, Extinguish: https://en.wikipedia.org/wiki/Embrace,_extend,_and_extinguis...
Slack advocates could say, "Don't worry, you don't have to switch. You can just keep using your IRC client with Slack. Everybody else will have a web UI, which they prefer, and you can keep using Irssi. Everybody wins."
Are we doomed as a community to be surprised every time a company with power squeezes?