You are about five decades too late in your comment. Without further immigration from the mother country to sustain them, the Euro-ethnic neighborhoods died off, some slowly (see South Philadelphia), some rapidly (see Irvington, NJ or much of Chicago's south side). As my father will tell you, after WWII, anyone who was upwardly mobile left the old neighborhoods, depressing property values, leaving behind only the old and the poor. In his case, new immigrants moved in.
The only reason certain nabes like Bensonhurst in Brooklyn remained Italian into the late 1980s was due to a wave of immigration that took place in the 1960s. Other "white ethnic" nabes that did not receive further immigration either "went black" or saw new immigrant groups come in from the late 50s/early 60s onward.
Of course, here in New York, new ethnic groups seem to move in every ten years. Many older "white ethnic" enclaves have been gentrified by yuppies/hipsters, but only after said neighborhoods went through a period of "new ethnics" living there for a time.