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.

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).

I think IRC is the Delorean of this analogy.

Slack is even easier to extend than IRC, and all those IRC bots can be plugged straight into Slack.

A sufficiently advanced IRC client may be able to catch up to Slack in some of its essential features, such as formatted code sharing, image sharing (for screenshots and other things), document sharing, comments on uploads. etc.

But at that point, why not just use Slack or Mattermost.

And IRC is not even close to an adequate solution for what Slack does when it comes to non-devs. If you only ever talk to people who code, then perhaps you can work around IRC's deficiencies. But by using Slack, the conversation can be opened up to all sorts of non-technical people, and it enables them to communicate effectively so much better than IRC ever could. These non-technical people's contributions are essential to the functioning of almost all serious companies, meaning that for most teams, IRC will not work and Slack will.

IRC is a telegram network operating on Morse code compared to Slack's cellular network.