Main problem with IRC is that it's not "always on" and I really need this feature. Sure you can use IRCCloud or SSH/Screen into a server, but they both cost money while Slack is free.
On a project a customer of mine disabled all those integrations because we couldn't go more than one week back into the past. Thery were not willing to pay Slack and we were losing track of decisions and accountability. When that happened we started writing down more things into Google Docs and the conversation shifted naturally from Slack to the comments of the docs. That had the (IMHO positive) side effect of reducing the time spent chatting about the project and of increasing the time spent to actually build stuff.