I think this is exactly spot on, and I see two societal pillars that can support these kinds of small communities: journalism and academia. Particularly in support of local or niche journalism, smaller communities can actually provide support and better reporting. And for academia, discussion is clearly a core part of learning.
I don't think existing startups can provide the infrastructure that's required here. I'm a bit jaded about open source at this point, too, but giving these institutions the building blocks is going to be important.
And the biggest thing of all: better hosting that actually makes all of this much easier than it currently is.