Re: Speaking as someone who knows bugger all about Slack..
It is not just yet another one. It is yet another one with a proprietary protocol.
I may not like XMPP as it is often way too verbose, but it works, interoperates and is proven to run and ludicrous scales (Google hangouts/talk and f***book messenger). I really do not see why an attempt to reinvent the XMPP wheel via JSON sreaming should be valued at 1Bn.