Zero probability of trade is indeed the feature which would make the argument in the OP potentially not go through, when irreversibility is present. (Though we would still get a weakened form of the argument from the OP, in which we complete the preferences by adding a preference for a trade which has zero probability, and the original system is indifferent between that completion and its original preferences.)
Zero probability of trade is indeed the feature which would make the argument in the OP potentially not go through, when irreversibility is present. (Though we would still get a weakened form of the argument from the OP, in which we complete the preferences by adding a preference for a trade which has zero probability, and the original system is indifferent between that completion and its original preferences.)