As far as the rest goes I've wondered about as well. I'm part of a Discord that has no subreddit, yt or twitch channel is not a gaming community coming from some game or the like. It has a focus in that it's Eurocentric tho there's plenty of people on it from the US and the rest of the world but the only way it has survived for bout 2 years now is by being welcoming and a very broad catch all community with channels for cooking, politics, tech, memes, etc (https://euro-lounge.eu) I've encountered other ones that survived but they tend to be part of some big crosslinking groups and seem void of community feel with a shitload of people 99% of which don't say anything. Meanwhile more interest-centric communities just die if they don't have an area like a subreddit to pull people from. I've encountered numerous disappointing graveyards like that.