On my last evaluation, I found the two most interesting to look at matrix and tox.chat. But in the context of a slack-alternative, matrix is definitely the one to look at. Also has an IRC bridge: https://github.com/matrix-org/matrix-appservice-irc

As for XMPP, I think it is still entirely viable as an alternative to using a brand new protocol and brand new clients and servers. But if you want solid off-line messaging, server-side logging and scalable group chat -- there aren't any out-of-the box combination of free/open servers and clients that will work, as far as I've been able to figure out. I assume one would want a solid command line client, solid gui clients for Windows, OSX, Linux/BSD, Android, iOS and Windows phone, along with a good web client.

I don't think you can pick-and-choose a XMPP server and clients that will provide all that, today. It is certainly possible to make that, standing on the shoulders of various open projects -- but the effort would be anything but trivial. I'd be happy to be proved wrong, though!

Unfortunately XMPP didn't take off. Notice how it's nowhere near as ubiquitous as email and web standards. Also notice how nearly all modern chat services don't support XMPP. I'd argue there are technical reasons for this situation. Try searching for "XMPP" in this thread for details.