I could (should?) have said, “the first obligation of a culture that doesn’t want to die out…”

Cultures do exist, they are a thing, they war, evolve, and die out. Almost all the angst we see in the USA today is about the internal culture war. Much of the foreign policy angst we have today is similarly rooted. Disregarding them as “not a real thing" is dangerous. Especially if you don’t want yours to die out.

a culture is an abstraction that describes a set of beliefs and behaviours etc. it doesn’t have any interests.

This critique appears to me to simply boil down to word choice. I maintain that cultures can definitely have interests, by this very common definition. If you disagree, that’s fine, but the disagreement is semantic.