But honestly, I think people will do better long term if they have to put in even just a little bit of legwork to find the communities with the right fit, and ignore the rest.

That kinda misses the point, though. For me it's more about promoting decentralization than it's about whether people's reasons to want to join all communities on a topic make sense (they actually can for niche topics). Without a feature like that, I fear people will just all join the largest community on the topic and "centralize" it.