Very interesting short essay. The idea that we are diluting the talent pool by allowing expansion is insane.
Besides the population numbers that you pointed out, people seem to forget that, unlike 1940, kids today are spending far more time perfecting the craft of sports. The average professional athlete today is infinitely better than the average athlete of the 40s and 50s in regards to the major sports (MLB, NFL, NBA). The time spent in preparation, training, and perfecting the craft is insane. Couple that with the evergrowing popularity of the sports and what you have, I believe, is a talent pool that is essentially bottomless.
Something funny, though.