Is that just down to immigration though? To me it seems that it is in part also because of the common understanding of what "being white" means: racial descriptions often still come down to a sort of visual one-drop-rule. If somebody looks even a bit off-white, they are seen as black. Common example here would be Obama, who is mixed race, but universally seen as a black man, even though thats just half the truth. With such an understanding of what it means to be black or white, a lowering percentage of white people is inevitable, even without immigration.