I concur with the OP, but I place an added restriction: Moral Framework only applies to those beings capable of voluntary action and reaction. In the absence of a network of decision making the moral framework would necessarily be reducible to that of strictest utilitarianism (do that which benefits the group according to whatever material, social, ideological, etc factors are being weighted for) since you would be dealing with robots or plants (things which are either in an ideal state or they are not).
Completely subjective Moral Relativism is a philosophical disease. It is one thing to suggest that ethical considerations are beyond the scope of human analysis (Meta-Ethical Moral Relativism: quite simply suggesting that such problems are unsolvable because our tools or conceptions are too complex for full understanding/analysis), but quite another to suggest that we cannot have any bedrock upon which a moral framework can be situated.
Survival of the Species is a biological imperative that all social systems are derived from. This means that to a lesser or greater extent this is weighted into all Moral systems whether we like it or not. Not sustaining the species equals dying off. End of discussion.
Selfishness is a biological/psychological imperative that all social systems are derived from. If we could not derive benefits for ourselves then as actors we would cease acting. Again, all social systems are derived from this, and that means moral systems are whether we like it or not.
Altruism is a biological/psychological imperative that all social systems are derived from. Biologically species will instinctively seek a certain degree of harmony in order to maximize self-benefit and survival of the species. And psychologically we are unable to exist in a vacuum of human contact (we go insane if we try). Thus, here again, all social systems are derived from this, and that means moral systems are whether we like it or not.
The issue is one of complexity (can ethical constraints that are consistent and applicable: i.e. will people actually follow them; be created or is it impossible) not existence. If everything was purely subjective, then no maxim could be generalized (and yet there are some behaviors which have been universally outlawed by every human culture on earth; precious few, but they exist), and attempts to generalize a maxim would always result in failure no matter the size of the population.
The latter issue (generalizing maxims resulting in failure) is the important one. Legislating morality is something that fails at the size of a nation. Prohibition and efforts like it have unilaterally failed across the world. So we know that generalizing morality across populations of a certain size fail. But we also know that playgrounds with small children manage to arrive a certain code of conduct (unspoken rules of interaction or precedence) that while not perfect (biases exist) are accepted by the vast majority or all of the "constituents." Populations of a certain size can accept generality without failure.
The issue is complexity: what size causes "rule induced failure?" and is it within our capability to understand morality as a totality?
MTF