This was always going to happen because Anglo countries define whiteness by one-drop blood quantum rules. The population of white people will always shrink if you define white in such strict terms and everything else in such loose terms.
Having just a single non-white grandparent is often enough to disqualify a person from whiteness.
It also doesn't help that the boundaries of whiteness have shifted compared to 30 years ago. Muslims usually aren't considered white for instance no matter where they're from. I distinctly remember my Syrian neighbors were considered white when I was a kid, then 9/11 happened and suddenly they weren't.

