Certainly the statement that collective moral constructs for the masses is an unattainable goal stays true for any attempt to impose morals from outside. However, morality when not imposed, could result in a construct collectively shared by a mass of people. In fact, you could argue that any common culture includes a moral construct shared by a large group of people. The real question then moves to the origin of such morality, and whether there is such a thing as natural (or inherent) morality, or if all culture is an artificial construct. Clearly Plato (through Socrates) believed that there was one single Good from which all virtue, and all morality derived (this concept also underlies the Cave allegory). That is, an absolute good outside of yourself was the sole benchmark of goodness. Thrasymachus started his assertions from a more relativist position, measuring the good from within himself when he claimed that justice is the advantage of the stronger (Republic 338) and that it is more advantageous to be unjust than just (Republic 343). Plato/Socrates’ refutation is essentially shifting the definition of good from personal benefit to absolute standard, and showing how in an absolute sense Thrasymachus is in error. Now if we accept the existence of an absolute Good outside of the individual as the source of morality, then a mass of people may indeed spontaneously choose a common morality without coercion, and you have a collective moral construct.