>This isn't the world of BBSes where everyone had a corner or two that they hung out on.

Yes it is. It literally is. If we want to do that we can just do that. The internet is not a zero-sum game. We don't have to play to "win."

>I don't think another decen-HN is likely to take off or break into other self organizing segments even though this is easy to attempt with some Mastodon servers.

The only reason HN is popular is its connections to Silicon Valley and startup culture. It didn't really carve out a niche so much as succesfully create and market an image. Without that it isn't anything remarkable, and there are already tons of technically-focused communities on the fediverse.