Re: Speaking as someone who knows bugger all about Slack..
We have an internal XMPP chat server, and some people in the company still wanted to use Slack. No idea why. Fortunately we seem to be standardising on XMPP.
And how did GMail invent threaded conversations? GMail's conversaton threads are the most confusing email interface I've used, far worse than the threading interface that has been common on desktop based email clients since the mid 90s.