What will we lose when we are entirely homogenous

When the population of earth has intermixed that they are entirely a single population what will we lose? what will we gain? I think at least religion will be dead, but think of all the IQ points we'd lose as a population

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I support a rich multicultural world consisting of mono-cultural nations that interact peacefully and for the common good yet protect their identities and cultures. I am a racist in the sense that people of different races should live in different parts of the world without interbreeding and genocide.