Edit: Discourse -> Discord
You have a great point about monetization. My understanding is that their monetization plan is to start selling things like sticker packs (e.g. how other messenger services get monetized), but I don't know how likely that is to succeed.
Having a mixture of groups is hard. When I was playing Destiny, my group there was actually using GroupMe (which is pretty awful for a variety of reasons). I actually created a Slack and wrote a GroupMe<->Slack gateway, though it kept breaking because GroupMe's API stopped sending messages until I cycled the connection, and I eventually had to shut that down when Heroku changed their free tier (which coincided nicely with me losing interest in Destiny).
My current WoW guild (of which I'm a casual, not raiding, member) uses TeamSpeak, and I don't know why they're not on Discord. It's annoying enough that I don't usually bother to join it.