My view is that subscribing to a community costs nothing.

It does, to an extend. People don’t want to subscribe to dozens of different communities to get their content. As I said in the other comment, there’s a balance to be found, and usually more generic communities benefit from not having to be promoted in a lot of places to get known, people just see them from All or a post in !newcommunities@lemmy.world and subscribe because they see it as active and generic enough.