There is a scientific argument for diversity. Diversity is absolutely necessary for life to thrive. Without diversity, living beings eventually end up dying.
If all human beings were genetically the same, we would all be equally weak to a common threat.
The Black Death killed 60% of the European population in the middle ages, but 40% survived because they were genetically immune.
Without diversity humanity would be extinct long ago.
With enough diversity, there will always be people able to survive, no matter the threat.
Diversity is the key for humanity's long-term survival and prosperity.