White Supremacists say the White race is being lost through interbreeding

Actually, they are right about this. The white race likely will be lost completely within 500 years or less. This is very likely. However, what they aren't mentioning is the White race the yellow race the red race will also become more or less a thing of the past. But, mostly this will happen to white people.

You might ask "Why?"

Well. If you study anthropology like I have it is about dominant and recessive traits.

For example, being white is a recessive trait which means if a black person and a white person have a baby generally speaking their baby will not be white but only a lighter shade of black. And even if the mother (or father) had blue eyes or green eyes or even violet eyes) the baby 9 times out of ten approximately will have dark skin and brown eyes because Dark Skin is a dominant trait and white skin, blue eyes and green eyes are all recessive traits unless the father and the mother have blue eyes and green eyes and white skin too.

So, 500 years from now likely the average person on the planet will not be white, nor black nor asian, nor red skinned. They will be darker than white people with dark eyes and their eyes might be slanted and they might have kinky hair. (But this will be the average person around the world). There might be enclaves of relatively pure yellow skinned people, and red skinned people and black skinned people and enclaves of white people with blue or green eyes or violet eyes.

But, these more pure breeds will be the exception and likely will be discriminated against by the majority who will be of all races and traits combined.

This is just what jet travel around the world will tend to do.

Why aren't white skin and blue eyes and green eyes a dominant trait?

Because they are an adaptation of darker skinned people so they didn't all die from not enough sunlight in cloudier climates.

White people are actually sort of like albinos in a way that developed when there wasn't enough sun to get vitamin D to stay healthy any other way where they were living in the far north. It also might have had something to do with their diets as well.