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

The dream of fascists was to make all people uniform. How enlightening it is, then, that the dream of liberals is the same: "Go interracial! Mingle, mix and melt! You don't have a right to preserve your cultural, racial, historical or national identity! You must become the same".