The argument against the measurement and existence of a good feeling definable as pleasure/pain and happiness is a strong one. Given that the fallacy exists, utilitarianism is the sect of positivism based upon both the positivist stance and the supposition against this fallacy. Positivism is the overall maximization of SOMETHING among all people. Positivism can survive past this impossible amalgamation of human experience into a more measurable, justice-oriented system, but utilitarianism, by definition, is stuck in a disproven state. Just abandon the term, and look to other plausible, positivist theories.
(To avoid bias, I disclaim that I deny both of these philosophical presuppositions)