A more central part of Harris’s argument:
…it also seems quite rational for us to collectively act as though all human lives were equally valuable. Hence, most of our laws and social institutions generally ignore differences between people.
Ah but they don’t. One big social institution doesn’t, at least not necessarily: the family. Some parents believe in equality, but some don’t; sometimes it’s a matter of what the male head of household believes, because that determines the rules for everyone else.
This is why the claim that maximizing well-being for all can be scientifically shown to be moral or good does not (as far as I can see) get off the ground. It’s because some people’s well-being partly depends on the subordination of other people, and people like that do not consider the de-subordination of “their” subordinates a source of well-being for themselves. Over time that can change, but it doesn’t happen overnight. So the question arises, how would you show such people scientifically that they are mistaken? It can’t be done. You may be able to show them evidence that the subordinates will have more well-being, but that won’t trump their sense of the fitness of things. The issue isn’t factual (though facts can help, or hinder; it depends), it’s emotional.