Immigration has very little to do with a population being homogeneous or not. Take the few extremely diverse countries with extremely heterogeneous populations:

Immigration has very little to do with a population being homogeneous or not. Take the few extremely diverse countries with extremely heterogeneous populations: Russia, India, Indonesia. These countries do not have a recent history of immigration at all. Now take a few counter examples: The USA and Brazil. Despite the vast size of these countries, and the high immigration rate, they both have a population that is rather homogeneous compared to what you'll find in the old world. Economically, the USA is more homogeneous still. I get the impression that to Americans the only measure for diversity is skin color, because you are used to everything else being the same.