It did last a long time, but opening their doors to immigrants from other cultures (my own included) sealed their doom.
As long as we had the melting pot, things were okay. During the immigration time-out from 1920 to 1965, people were assimilated into an American culture. People identified as Americans.
Now though, we have to bow down at the altar of diversity, which, far being a blessing, is a force for disunity and violence (not to mention expense!).