You are implying that a child's need for [...] stable caretaker figures, guaranteed not to disappear no matter what happens, is a learned cultural thing.

Not exactly - I'm implying it may be a contextual instinct. That is, a highly nuclear family pushes different buttons from a highly extended one.

You also haven't addressed the feeling [...] knowing that anybody could at any time choose to abandon you

The feeling of being a rolling ball on a narrow hill ledge is different from the feeling of being the same ball in a valley bottom. Children would tend to fall out of unstable families and into stable ones. Having lived my childhood in an unstable family, let me assure you that the feeling "this is teetering on the precipice" is not assuaged by the inability to swap.