This is why open source for communication platforms is so important.

Discord WILL disappear at some point and millions of people will lose their communities.

Discord's just a platform. When Discord will disappear, I don't think it would happen overnight and the communities would have time to decide where to relocate, hopefully for an open-source self-hosted solutions, but more likely for the next hot thing in instant communication. And it's not as if communities don't move from platform to platform already: like wasn't there a big wave of people moving from Digg to Reddit a while back?

Online communities are far more transient and far less effective than ones rooted in geography. I'm not saying that they don't provide value or aren't worthwhile.

Online communities can rebuild quickly are more resilient in a sense e.g. Digg to Reddit migration.

for discord emigres, teamspeak still exists, and for social media all you need is an old school forum that hosts videos and voila