Why would I forego the use of a tool that gives me dramatically more productivity right now than I could get even spending 100 hours configuring and managing an IRC server on the prediction that some years from now Slack is going to suck?
I understand using IRC because of privacy concerns or you have very specific workflows, or just because you like managing your own services. But the threat of future changes is not a credible reason to avoid Slack. When it starts to suck, you just switch.
It takes less than an hour to set up an IRCd, even with SSL.
Slack has a bunch of advantages over irc, though: clients for many platforms, connections go over HTTPS (non-issue for firewalls, whereas IRC connections are sometimes non-trivial), cross-platform notifications, inlining of some content (slack calls this "unfurling").
Their trump card, however, is the simple third-party integrations support. In ~30 seconds, I can write an integration that e.g. watches the git repository on a machine and sends a message to a specific slack channel when the HEAD changes.
Yes, you can do this with an irc bot as well, but: you have to get that irc bot (maybe it's a library, a binary, whatever) to where you want to run it, and you have to write a few lines of code. Whereas with Slack, you can just curl an https endpoint from bash. There's no need to deploy anything, it's all there, and it's simpler to use.
I've seen less-neck-beardy teammates throw together slack integrations that have helped out quite a lot.
Slack lowers the bar to not just team chatting, but making the team chat work for the team's environment. In that it's a bit like emacs. It lets the team change it to fit their usage.
I understand using IRC because of privacy concerns or you have very specific workflows, or just because you like managing your own services. But the threat of future changes is not a credible reason to avoid Slack. When it starts to suck, you just switch.