I thought about this a bit. I'm a huge opponent of closed source / walled garden-style approaches for many platforms and situations, and I'm curious why this seems different for me. The biggest difference between Slack and normal situations where you do not want a walled garden is that porting between chat programs is relatively easy. Unless you're highly invested in the user flows allowed by a particular chat application (bad idea, IMO) which particular client you use doesn't matter that much. The only thing I can see this being a problem for is chat history, which you can still access from the old platform if you need it.