[krapp]

>So in theory the modern day could be a Cambrian explosion of tiny communities. Is this the case?

Yes, it has been.

>Maybe there are a lot of them hiding and they fear eternal september?

They're mostly on centralized social media platforms like Reddit, and discovery of niche content is more difficult on the modern web, but that doesn't mean that content isn't out there. I'm using "community" here to refer to a group of people with common interests, not an individual website. So "Reddit" is not a community in and of itself, but /r/niche_subreddit_youve_never_heard_of is.