The issue then becomes "well why don't they just go back to a Teamspeak server? they can self host it!"

But we're forgetting there that the average person online is not a dev. The most they usually know is how to point and click on something. Which also means they usually don't know how to spin up a Linux machine/VM somewhere and install their own chat server.

Discord is popular because it lets almost anyone on Earth point and click to create a chat "server". If someone can figure out how to do that (eg cPanel), you can absolutely break their moat.