This is very much untrue. Both the Irish and the Italians were on the bad end of this behavior recently. And they retained their own culture for generations - some of it still exists, like the fact that there are a load of Catholics in the northeast - that's not from the initial bunch.I have to disagree. That's fine if you don't intend to have much immigration.
No one cares about having a Chinatown etc It brings flavor and nice food. However when there's mass immigration you need to integrate and America has always in the past tried to spread around it's new immigrants to aid in this. It's only recently that no longer happens.
That's without touching on how much East Asian immigration has impacted the west coast, or eastern European immigration more recently to New York.
Assimilation has ALWAYS taken generations, and has never been complete.