what about Roman families?

https://en.wikipedia.org/wiki/Family_in_ancient_Rome

You had the familia, which was similar to the current nuclear family, but that was wrapped into the larger gens

https://en.wikipedia.org/wiki/Gens

people cared not just about the success of their intermediate family, but also their gens, which was similar to a clan.

You'll have similar structures in many tribal societies.

Do you have actual statistics to support your hypothesis that

> My read of history is that it’s the single most stable and ubiquitous human social arrangement by a very long shot.

besides just "oh yeah bro, it's my read of history bro, totally rigorous"?