I can understand that sentiment in a nation that is mostly homogenized such as Japan. Other than China and Korea, Japan hasn’t had mass migrations to and fro.
The United States (and to a lesser extent, Europe) is not like this. The US was a land of indigenous people that then came to include Europeans and Africans. The ‘union’ of the US, however, has never been one homogenized culture or people. It is slightly reverting to an indigenous group with the immigration of its neighboring countries, but they will likely never be the dominant group in this country.
Europe as well has always had migrations throughout, especially the southern and eastern parts. But even the UK territory, during the Middle Ages, was already an amalgamation or different groups of people.