The main issue with IRC is that you have to stay connected all the time to recieve messages, and you have to leave your IRC client to see and search the logs (unless you have some intermediate IRC client server that runs all the time). It's just not as nice.
Yet it's not something that IRC provides out of the box.
I've got no issues running my own ZNC bouncer, but when you're choosing a product that an entire organization will use (including people who have no idea what "ZNC" means other than 3 letters), IRC is lacking a lot of features that get mentioned in every HN chat solution thread.
Yeah but if you suspend your machine doesn't it stop accepting connections? I personally set up a tmux window on a raspberry pi for this, but it's too much to expect a whole company to manage this (vs. a nice simple web GUI like slack)
Slack has seriously changed the way we work at my company and it's boosted our productivity by 100x.
Yes we had a IRC server for a while and no one was using it, so we always had to use emails for important communications.
You just got to try it to feel it. Some features on paper don't match with the experience of a product.
If you're worried about using it for large community projects (e.g OSS), then maybe IRC is still the better child for that particular audience, although there's already a couple of communities who have successfully moved to Slack entirely (Gopher comes to mind).