The issue is that a mixed white/black person is perceived as black and not white. The one drop concept still has ripples today. So yes, eventually the whole earth will begin to homogenize into one "race" and the thought of all those mixes let's us see how ridiculous the concept of race is. Since white people are a global minority, they will be one of the first major "races" where we can see a decrease in white populations(depending on how you define white.... how white is white enough to be white? One drop? 25%, 50?). By 2042 roughly, there will be more POC than white people in the US, but many of the contributing population to that stat will be children.
Ultimately we really shouldnt care, the only loss would be the loss of white cultures(bcause theres nothing innately better or worse about being white or non white), but an Asian person with an Irish grandma can keep the culture living, for example. So losing white skinned people isnt rlly bad or good it just is.