What matters, surely, is per capita GNP growth, not simply GNP growth? However,  McKenna doesn't specifically address per capita GNP growth.  India and China have much larger GNPs than Canada, but much lower per capita GNP. Lots of  people most of whom are poor tend to produce that mathematical result. Trump's proposal, as I understand it, wasn't to reduce the number of skilled immigrants to the US but to reduce the number of family category immigrants (as opposed to just spouses and underage children). One might plausibly argue that increasing the number of skilled immigrants even more would increase US per capita GNP and would be the rational thing to do   However, it is less clear how admitting fewer aged relatives would decrease per capita GNP,  even in a country that, unlike Canada, lacks universal health care, or, for that matter, how admitting fewer aged relatives would decrease the number of candidates for low paying unpleasant jobs.