Jews are a classic case in point
All major ethnic groups in US - be it Germans, Irish, Italians - brought in their unique genius which was an outcome of endogamy - which typically lasted for 1-2 generations
But diversity is most beneficial as well as most problematic only when it exists in the same geography
Let's take the example of Marwaris - a very mobile group with significant presence across provinces. From Kozhikode in Kerala to Kolkata in Bengal
Endogamy is practiced, and a certain cultural distinctiveness remains even 100-150 years after migration
Even a millennium later, they retain their distinctiveness, and dialect. And their outcomes are different
They still speak Tamil and not Kannada, even 2-3 centuries after migration
But it is not altogether a negative thing - as it is a source of cultural capital, a counterweight to populism, regional nationalism, and groupthink
It is unlikely at least in the short to medium term - say a couple of generations