> Given A and B, if A were good enough, people would use A... What if B is better? What if both are good enough?

Prior to Slack existing, lots of people who now use Slack weren't using IRC, because Slack offers a lot more.

Everyone I know working in academia or on a startup that uses any kind of instant chat uses Slack. None of them previously used IRC for the same purpose.

And it's just _obvious_ why, once you've used Slack for a while. They're quite different -- IRC is a protocol for real-time discussion on the internet. Slack is a business tool that incorporates real-time discussion.