" If, for instance, humans would be generally happier and suffer less in a social democratic society than in a fully libertarian society, the former would be preferable to the latter."

It seems to me that we generally reject such reasoning as a justification for coercion in private life. Suppose you and I and half a dozen others agree to meet for dinner in a nice restaurant. The bill comes at the end of the meal, and as it happens we have no previous agreement about how the bill will be paid.

Someone points out that Bob is wealthier than the rest of us, and argues that Bob should pay the whole bill since the price of the meal has a lower marginal value to him than it does to anyone else at the table, and thus having Bob pay will maximize happiness and minimize suffering at the table.

In private life I think almost everyone would agree that this reasoning puts no moral obligation whatsoever on Bob to comply, and that it would be wrong to compel him to pay for the others even if that maximized utility for table. Bob seems fully justified in paying for what he ordered and then going about his business unhindered.

Given that we reject this kind of coercion in private life as unjustified, I think there is a substantial burden on the statist to show why similar coercive behavior is justified when performed by government.