The U.S had African slaves, Chinese railroad workers, Native Americans & Mexicans from conquered territory. If the U.S really wanted to maintain some sort of ethnic homogeneity, then not expanding past the original 13 colonies and abolishing slavery right away would have been the way to go.

But that didn't happen because the U.S had already gone into Empire mode, and sought to expand its borders into territories that where clearly non-white, while also importing more of them for cheap labor. The end result couldn't have been more obvious.

Meanwhile, look at Iceland, Poland, Estonia and several other European countries. They have Jews but are still practically ethnostates. Although it also means none of these countries are also going to be wealthy as the USA, ethnic purity comes with a trade off. The countries that want to be super rich and welcoming were involved in diversity and imperialism at some point.

Edit: It's also the same reasons too why France and the UK have caught themselves in similar situations, although their changes in demographics are not as dramatic. For thousands of years, they barely had any contact with non-whites, even within their own continent. But once they sought to expand and conquer them, it's no surprise many of these ruled over subjects now feel entitled to move back, especially with the politics in their home countries being intentionally meddled with by these powers.

Like the Hong Kong situation is a perfect example of a territory Britain should have never taken without the foresight they were going to give it up anyway, but now the natives living there would rather flee than live under Chinese official rule.