this is just my impresson from what I've read but what you are missing now is the urban component that always was a main source of talent. In the late 1800s through the middle of the last century, there was a heavy urban Irish-American presence and by the 20s through the 50s and 60s, there were other Americans of European decent coming out of the cities. We are all familiar with the decline of African-Americans in baseball and the establishment of the RBI program to pick that up. Out of urban areas, I think you now get some Hispanic-heritage players, but, other than that, my impression is that the feed to minors and majors is more rural and suburban.
The feed from the Carribean and Central and South America has expanded, but there always was a presence. We've more or less swapped Cuba for the Dominican, and while the number of Puerto Rican players has probably declined, Venezuela has picked up.