I don't exactly disagree but you're upping the subtlety a bit. You're arguing that people should not regard certain things as terminal values when making political decisions, things which I think in fact they do regard that way. But if I get to dress my sockpuppets up a bit I can have them say, "Fine, I agree that I'll be a good utilitarian and try to maximize population happiness over time - but I think the long-term benefits of nationalism/integrationism clearly outweigh the marginal effects of short-term economic developments." Then someone could argue against that point - and we would almost certainly have improved the political discourse. But the people with the original elaborate analyses of short-term economic effects would still have been just as wrong in their claimed superiority.

In any case, the general point I wanted to make can certainly be made in a formally correct way as long as you don't insist that every rational intelligent being must have the same utility function.