I have believed in many theories about the attractiveness of breasts, including the nursing angle. Nowadays I mostly believe it has to do with what you mention: a woman who likes and enjoys her own breasts and cleavage will present them in a way that is attractive, and at the same time appear to be sensual and unafraid. A woman who feels bad about them will hide them away, and seem demure, maybe standoffish.

A man might still not be attracted in the first case, or attracted in the second, but I believe that in a lineup most men, if asked to speculate about the women's traits, would associate a significant cleavage with a woman who is at least one or two of playful, agreeable, (possibly) kind, centred, affectionate, sensual, and confident, and a buttoned-up chest with a woman who is serious, standoffish, insecure, and demure.