Utility maximization is a proposition that can neither be confirmed nor refuted. For one thing, "utility" does not even exist as an entity; it's a mere conceptual construct. For another, "maximization" as a motivation is inherently unobservable. Even behavioral economists, who are adept at finding so-called empirical "anomalies" --- behaviors that apparently contradict economic predictions --- talk of "rationalizing" those anomalous behaviors by coming up with models in which the anomalies are outcomes of some maximization process. See, for example, Thaler's review of the literature on ultimatum/dictator games which he pioneered.
What the experimental results from ultimatum/dictator (and trust) games tells us is merely that people are not income maximizers. It is nevertheless invalid to conclude that people do not seek to maximize some other criterion or objective, which may or may not be part of what economists call "utility".
As economists, we understand that the specific measures we use for utility, like income, satisfaction, or selfishness, are adopted because of analytical convenience for the problem at hand, not because we seriously believe that those measures reflect human nature in any deep philosophical sense.
In a way, it's unsurprising that no economist, famous or otherwise, would declare utility maximization "wrong", since doing so would first require a precise definition of what "utility" is. Such a definition, despite nearly two hundred years of intellectual endeavor, remains frustratingly elusive.