If Discord can keep up the growth, things like stickers, emojis, and sound packs could pull in good revenue. Also, the gaming community is already used to paying for these items on sites like Twitch
I have to admit, Slack and Discord are much nicer to use than IRC - even something like IRCCloud (and I'm a relatively happy paying customer of IRCCloud).
It's probably due to location. LINE is absolutely dominant in Japan, Taiwan, Thailand and a few other Asian markets. None of my friends in America use it really. Facebook messenger is clearly watching LINE closely though.
It's not really totally separate. Discord is a web app and you can run it in chrome. The only feature that you lose is push-to-talk, last I looked.
Granted, a lot of people will run the electron-based desktop app (which packages the web site), but IIRC, there's a plugin for twitch emotes for it too.