Earlier quoted context omitted.
IRC is just a well-established and battle-tested server/client protocol. I'm eager for the day when a competing open source server/client protocol emerges and takes over. I imagine all it would take is a really good reference implementation server and client that everyone can just start using, right?
I certainly think having an open source reference implementation for a 21st century (and by that I mean having features like Slack or Yammer or other popular closed source packages) implementation of a successor to IRC would be the minimum starting point.
http://www.matrix.org/
There's also Zulip, which as far as I can gather, is battle-tested, but does not have a strong story for federated servers, nor a good out-of-box experience for really small servers: https://github.com/zulip/zulip
Finnaly there's https://tox.chat -- which doesn't have a lot of the things IRC/Slack has (it's focused around p2p chat) -- but extensions are planned, for eg. persistent group chat. Perhaps most excitingly "new" of the three, which predictably is both a good and a bad thing: https://wiki.tox.chat/users/faq#what_is_tox
For those that "want Slack", but self-hosted, open/free software server -- I think Matrix is the most viable alternative -- if IRC is seen as not good enough.I've not included any XMPP servers, although eg. Prosody should be simple to set up and use -- because, apparently like IRC, it has too many problems for people to actually embrace out-of-the box XMPP for team chat. The fact that most big public services that host XMPP tend to favour anonymity probably has something to do with the fact that getting reliable server-side message logging and off-line messaging is still not as easy as one would expect, for any(?) of the big free XMMP daemons (or indeed support client side).
[ed: Matrix also have some support through bots/plugins for IRC bridging: https://github.com/matrix-org/matrix-appservice-irc ]