My guess is that Charles Kenny is agnostic as to whether it would be a good thing for the U.S. to extend its global dominance. But he argues that the best way to achieve that end would for the U.S. to increase the size of its population:
If more labor is the secret to continued American global economic dominance, there is one course other than importing it — expanding domestic production. Paying people to get pregnant can work. But, frankly, this is a case where there is a huge advantage to outsourcing. Domestic production is very high-cost. The average expenditure to raise a child to age 18 in the United States is $227,000, according to the U.S. Agriculture Department. That’s the same cost as raising 34 kids living on a dollar a day to their 18th birthday in the developing world.
The migration policy that would be most likely to contribute to continued American global economic dominance would presumably be mindful of the net fiscal impact of the migrants in question, e.g., there would be a much heavier emphasis on encouraging the migration of skilled workers who are relatively young. Similarly, the average expenditure to raise a child is a somewhat less useful metric than the lifetime net tax rate. The lifetime net tax rate for an individual who settles in the U.S. at age 25, having received the bulk of her education overseas, might be lower than for an American, who consumed various public services as a child — but these trajectories will diverge if the American earns a high lifetime income while the immigrant does not. This is a rather unsentimental way to discuss the underlying issues, I’ll admit.