One thing that makes race mixing good over the long haul is because of exposure of bionanotechnology in your body that hasn't seen the light of day for the last 700 million years because you're not a rodent living in dugout caves anymore.
You have a shit-ton of chemical process redundancy in your body, for example the process by which your digestive system can convert one thing into another thing usually has like 3 to 7 layers of redundancy. Take caffeine for a fun example, all peope have 3 known seperate processes for downsampling those imposter adenosines back to inert non-neruotransmitter status, when you hybrid with a person who has been living on Sentinel Island in the Pacific for he last 750 thousand years and have a baby, you have three or 5 separate redundant systems but the other person has none. The hybrid baby comes out with a downsample of those redunant systems, giving more volume space for other more important bionanotech subsystems to express themselves and evolve a domination over tasks that do decide fitness today.
https://www.youtube.com/watch?v=xoqyF6DJZ-YCheck it brosephs, Race mixing is good, because it gets rid of things that doesn't matter, and creates space for new things to be created that might make you the standard for humans over the next 15 thousand years.
Check it, the calculus works out.