The United States' population has grown by about 40 percent since 1960. And MLB now draws on the huge talent base in Latin America and Asia. Since the ratio of ballplayers to population is no larger than it was in the "good old days" I see no reason to think that talent is diluted.
(Not an original idea of mine - got it from Bill James).
If you really want to see evidence of this, just look at Major League Baseball today and notice how few great black American players (as opposed to dark-skinned Hispanics) there are in the game today.
There's a lot of truth to your statement about Latin American and Asian talent comprising a big chunk of MLB rosters these days, but that doesn't necessarily bode well for big-league baseball. I've pointed out that this trend may very well relegate baseball to the same status as soccer among American sports fans.