When my parents grew up in postwar NYC, they were part of a fairly insular Irish-catholic community. Sundays at church, weekdays at the church school. The church you went to was dictated by your country of origin. They talked to the Italians, but eyebrows were raised if you were hanging out with the Polish or god-forbid, some protestant.

Prosperity broke down the ethnic neighborhoods. This still happens today. Talk to Chinese kids raised in America what the family dynamics are like when you are an "ABC". Ditto for children of Indian descent or really any group with strong ethnic identity.

You see continued strong ethnic clustering where the ethnic identity is also associated with business and prosperity. Greek Orthodox communities are good examples of this.