And I'm on the other side of the issue because I take issue with his reasoning. So what?
The point being that his reasoning is fine. The idea of you taking exception to it has no bearing on wether his reasoning is failing.
It would be like asking Michael Moore to give a balanced view of conservatives in his next movie.
IMHO Sowell's logic is, well, just what you expect from Thomas Sowell - flawless.OTOH it doesn't address - nobody seems to address - the real issue. The real issue is how immigrants are brought up. We don't want Islamists, to take the most obvious example.
But even without that concern, there still is a difference between a Canadian and a Mexican. Although both countries inevitably (in the nature of things) define themselves partly by not being American, they still are quite different from each other. A Canadian emmigrant to the US will almost inevitably be a native speaker of English, albeit with a Canadian rather than an American accent. And Canada is not noted for the absence of a middle class, as is essentially true of Mexico.
And not only do Canadians speak English and belong to the middle class, Canadians don't come from a culture which still resents being defeated in war and dreams of reversing the decision of that war.