Not true. Multiculturalism DOES affect specific cultures. It actually acts as a "culture shock." Case in point: Muslims in Minneapolis use loud speakers to announce their "call to prayer" which disrupts the lives of people in the neighborhood who are not Muslim.Don't miss the point. Lots of other cultures being here does not diminish American culture. American culture is not just one thing, no culture is. Cultures are not fixed items. By definition they are fluid and constantly changing. This reality is a large part of how our Melting Pot works.
I'm sure you would agree that the Christian culture brought to America by Europeans disrupted the various cultures of the American Indians. The new government's answer to this clash of cultures was to separate them by creating vast reservations whereby the Indians could preserve their own culture without blending (or bending) to the Christian culture of the Whites.
When the Jewish Bolsheviks toppled the Russian aristocracy, the Christian culture of the Russians ceased to exist as it was displaced by the new, Marxist government.
When you pour a glass of fine wine into a glass then mix it with Budweiser beer, you no longer have a glass of fine wine nor do you have a glass of beer. You end up with something that doesn't taste like wine OR beer and I doubt it tastes very good.