Depends on how much funding Discord has already raised, I bet. Their VCs are generally going to want a massive exit, and I bet the valuations after previous raises are non-trivial. Guessing that Valve is adverse to acquisitions in general too, for their weird cultural reasons.
Twitch (and thus Amazon) is currently making a big play to compete head to head with Discord. At least among the people I talk to, it's an abject, miserable, flaming disaster, but I think it's a bit early for Amazon to walk away from it just yet. And they might yet pull it off; they have the resources.
EDIT: $725 million. Not cheap.
https://techcrunch.com/2017/06/07/discord/