I fail to see the connection between more immigration and improved utility.
If I didn’t think I’d have higher utility in a different country than in mine, I wouldn’t want to move to that country in the first place. Granted, my moving there could affect other people’s utility, but it’s not obvious a priori what the sign of the net effect would be. EDIT: Most of the obvious externalities are pecuniary, so their effects on total utility cancel out when considering both sides of each transaction (assuming they have equal marginal utility for money).
If I didn’t think I’d have higher utility in a different country than in mine, I wouldn’t want to move to that country in the first place. Granted, my moving there could affect other people’s utility, but it’s not obvious a priori what the sign of the net effect would be. EDIT: Most of the obvious externalities are pecuniary, so their effects on total utility cancel out when considering both sides of each transaction (assuming they have equal marginal utility for money).