There are so many open source solutions, Mattermost, Rocketchat, etc, why are companies willing to pay $100k/yr for Slack? What was the defining feature that others didn't have? Even Discord feels like it has far more features than Slack.
I love IRC and XMPP. I'd love to run one of those, or some new service (Matrix?), at work. However, my time is arguably better spent doing anything _other_ than maintaining such services, and the same goes for most engineers at most companies, sadly.
Side factor: the mobile clients for IRC and XMPP almost universally suck, at least on Android. I imagine if those problems had been solved in a reliable way, more companies may consider them (assuming the allocation of engineering resources problem isn't a problem).