What is a "reasonable GitHub alternative" exactly?

You can literally take the exact same software that Codeberg uses, Forgejo, and stand up your own alternative with different policies, with the only real cost being operational in nature. It seems less the case that it's a "low-cardinality space", and more that people feel entitled to other people's labour under their own terms.

GitHub, Gitlab, Bitbucket, Codeberg, SourceForge, SourceHut, and that's just from the top of my head. That's not what I call "vanishingly rare"

They may be troubled but Codeberg owes them precisely nothing