Amazon doesn't need to buy Discord because Curse/Twitch has close-enough feature parity and if they successfully get enough people to install Twitch Desktop for some reason (there are reasons more than just chat, like streaming), some portion of people will switch or run them concurrently. They don't need to hurry, they can siphon marketshare from Discord passively instead of actively.

On the other hand, this move makes Discord an attractive acquisition target who's looking to counter Amazon by pursuing the exact same market with a very similar integrated offering, and who thinks they can buy the market- and mindshare without users fleeing afterwards.