Discord might have a lot of Nitro sales, but that's probably dwarfed by their CDN and bandwidth costs of all the streaming that goes on with it. The infrastructure to relay massive scale of streams (even without transcoding) is not cheap. There's only so many streaming minutes at 720p that a single $9.99 nitro sub can support.

They also do gif transcoding and a bunch of other things that do take actual compute resources that cost money.