They had a really well defined user-space, marketed at it well, and really nailed the user experience, while still being free for the typical user. There is a lot to love about Discord.
I feel like everything down with Discord could be done with IRC in a open source way. IRC for the 21st Century?
Edit: Matrix already works great as an IRC bouncer for networks which have been bridged.
IRC for the 21st century IS these apps like slack, discord, etc.
Bundling up everything into a centralized, proprietary package will go bad for the consumers eventually. But for now they'll trade away their freedom for convenience and enjoy it.
IRC is a fine platform. That doesn't mean it's what everybody needs/wants.
"wait, why are you walking away from me, i'm talki-"
Then again, using a monospace font like I did for years (ssh+screen+irssi) wastes a lot of screen area just because every character is so wide.