Well obviously America wouldn't be what it is without people from other countries. This country is made up of people from everywhere, and everyone influences it in some way.
Some may prefer to keep to their communities, but even that has an effect as others enter that community to find shops and restaurants. And through the generations people usually end up blending with the rest of the country, with means we all learn from them and change, as well as change who they are.
It may not make everyone happy to think that their culture will get watered down over the years, but without that why be here in the first place?