I think that model is more sustainable, if we do assume that discord will sunset eventually I would guess two possible avenues. First one is feature creep - different communities want different features and eventually discord forms into everything app that new users find confusing. Maybe there is a new simple, cool, and hip competitor that takes off. Second one is rotting of each community from the inside by overwhelming them with information. I've gone through plenty of servers that I've fully engaged with, but that had grown too big. These servers are on permanent mute and I never read any messages because keeping up with the traffic is impossible. The result of this is that instead of keeping discord running in the background I switched to opening it only when I want to game with friends.