My spitball opinion on that: we had diversity in America during the beginning of the industrial age that we heavily relied on for it's host of benefits, innovation primarily as well, but it didn't create the conflict (conflict theory terms) we see today because you were expected to integrate (conform) to be "American", which was a shared set of values and beliefs. America is still today one of the few geopolitical distinctions today that is solely demarcated as such by subscribing to a fundamental set of beliefs, rather than by blood as you have in most other countries in the world, especially in Europe. America is chiefly an idea, and by believing in that ideal, you gain the entire inheritance.
This is something that is continually being eroded unfortunately, especially as we continually descend into more and more hyphenated subgroups of Americans, rather than just focusing on what makes us simply Americans, but something we should still strive for.
(And I know you will get downvoted for posting about Robert Putnam research here because it appears to come off as a knock against diversity, even though Putnam himself said his research actually affirms the benefits of diversity)