Given A and B, if A were good enough, people would use A... What if B is better? What if both are good enough?
You go on as if your first sentence stands up on it's own. It makes no sense at all.
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.
Wow, going off the deep end here. Unless you are a time traveler, there is no way to not use IRC because of slack, 'prior to slack existing.'
Uh, nope. Most people use communication platforms due to network effects: because they were invited to use it, because their friends/coleagues use it. Slack is no different.
I mean, Slack appeared on scene after many other technologies existed. People recommend it for their friends because it works well. There is no global network effect at play.