Without a race, there will be no individuals of that race, optimal biological partners are third and fourth cousins, see "An Association Between the Kinship and Fertility of Human Couples", they have the most healthy babies. Outbreeding is breeding without one's "race" which could be defined to be really 6th cousins and beyond, and it's a thing that can be as genetically dangerous as inbreeding which is breeding with siblings and 1st cousins.
https://science.sciencemag.org/content/319/5864/813 I think the idea it is possible to blend all races successfully without simply causing mass unfitness and mass defects in anything less than ten thousand years is mere imperial delusions of grandeur spurred on by madmen's pride. nuntius interficere non, fortes fortuna iuvat
