As you noted, one of the wonders of capitalism (or free trade, or the right to transact, or the right to refuse to transact) is that every voluntary trade increases net wealth. Only when both parties are made better off does voluntary trade occur. This concept is very mysterious without the understanding of subjective preference.

My objection is only to the specific justification of progressive taxation based on interpersonal utility comparisons. There are other justifications of progressive taxation not subject to my objection.