Hey, have you looked at https://matrix.org? If you build on us, you don't have to fear ever being cut off - we're completely open source and free! We do also have a slack-bridge, and we're building more bridges - our goal is interoperability rather than isolated silos!
As a startup founder – chatlio.com – betting on the Slack "platform" I have to say that it feels pretty great to know that they are not planning on cutting us off tomorrow morning... :) Might happen anyway of course, but this announcement means we can work somewhat more serenely for a while longer. I see this as a kind of "stability pledge" to third party dev shops.