Unlike in 1900, we now have illegal immigration. Simon does not address this question. His apparent object is solely to encourage complacency about the status quo. He dismisses this as irrelevant to economic policy. Wrong again, of course.
National origin correlates powerfully with economic success.
First World immigrants generally outperform Third World immigrants by a factor of as much as ten. Simon remains fatally dependent on old data. This is an acute problem in contemporary immigration research. The 1965 Immigration Act did not take effect until 1968. It was years before the changes it wrought began to show up in Census data, and even longer before academe confirmed them.