I tried XMPP recently and was surprised to see just how barren it was. The tech might be good but if nobody is using it after all these years, what's the point?
I think this will be a pretty similar case because discord straddles "small personal servers with ten friends" and "large official servers with 500 users for a particular game" and "tech support forum for an open source project", and one user might be in all of those servers pretty easily.