Why can't the society have evolving rules? Looks like your "rules rigid enough to structure a society" phrase denies this is possible, for no reason.

I was likely being too definitive there. I think there is a strong tension between rules rigid enough to structure a society and maintaining a principle of openness. I think optimistically allowing for failures of various kinds (one could imagine it at a lower level than the entire society) allows for a more robust pursuit of openness.