Inviting different genetics was probably essential to prevent various diseases related to incest. That's what happens when blond/e Aryans only have sex with other blond/e Aryans for several generations. They might be retarded, but they did something right. Their problem seems to be a broader inability to understand things like "unintended side-effects" of their control-freak desire to make their society run like a machine.
For the past 30,000 years, Europeans were doing quite well without any infusion of the other races. Ethnic Europeans have one of the lowest rates of genetic disorders in the world, when compared to the other groups (a few groups, such as the Ashkenazi Jews are an exception). This means that the European genetic pool is diverse enough.