One of the reasons I can support race-mixing is because it allows desirable traits like  intelligence to flow into our race, which gradually brings down prejudic. As I mentioned previously on this forum that the reason why whites can't create their own 'thriving' civilizations is because they produce too much propagands against multicultural people which puts a strain on diversity. When there's more racist people in a civilization than accepting people, more resources and attention need to be put towards taking care of each other; people will be available to create institutions which holds less stigma and widens opportunity for all backgrounds.