> American society is high trust and diverse

Would note that this is almost a prerequisite for great societies. Small and homogenous, or powerful and diverse. There really isn't a middle course.

Rome. China. Britain. Each had empires that were remarkably diverse for their time. (Rome, perhaps, most of all.)

I'll disagree with that on a few points. Britain was inarguably the most diverse, with almost no attempt at creating cultural homogenity (and it really wasn't that great for anyone not Scottish or English.) Rome attempted it to some degree, with attempts to have a unified culture largely decreasing as time went on, being replaced with a Christian identity. China is extremely complicated in this matter and goes to extreme lengths to ensure cultural homogeneity. Minorities exist and the nature of the han ethnicity is also very obtuse, but it's highly rooted in an indivisible homogeneity that you discount. Hanhua is a very basic concept.

You also ignore the flagrant existence of powers that were not diverse. The Phoenicians interacted with a lot of cultures and influenced them very deeply, but Canaanite society was highly insular. Viet Nam was a powerful society that expanded continually, but it engaged in aggressive replacement colonialism of peoples it conquered.

Diversity in a real sense requires a collection of disparate, conflicting identities. There are no great societies that have any kind of lifespan with widespread diversity in this sense. Almost all of them move towards assimilation, and the ones that don't never last long.