The IRC gateway was there to mollify people who said "IRC is better! I refuse to switch!"
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."
Right, and now that everyone is hooked the loud minority (if we're being honest with ourselves) have to choose between speaking the Slack protocol or losing access to our contacts.
Are we doomed as a community to be surprised every time a company with power squeezes?
Embrace IRC. There are some excellent IRC clients, such as IRCCloud[0]. With web & mobile versions. I'm on IRC all the time, and there's still lots of good stuff there. I've been an IRC user for 15 years, and haven't lost any contacts due to arbitrary corporate decisions.
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."