re: Is there any historical evidence of diversity working out?

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it was close to 90% white

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Diversity is not solely a racial issue. It is culture, religion, educational, etc. Not remotely.

E.g., in the nineteenth century, there were leagues of difference between German, Italian, Polish, Scandinavian, or Russian immigrants, all "White." American exceptionalism tapped the best of each within a social melting pot. We took cultural nuances and improved them.

American pizzas became something far better than their Italian forebears. Tacos, English muffins, french fries, were all formed from the same basis, yet all 100% American. E.g. the first "English" muffins were sold in England in the 1990's. They are purely American adaptations of crumpets.

I suspect your reference leans more to racially divisive efforts of neo-Marxists. With their influence, leftists now consider an inclusive adoption of things like Cinco de Mayo, Native American art or garb or nomenclature to be "cultural appropriation," and highly inappropriate. It is the literal antithesis to previous diverse inclusion directed toward a common goal. The leftist effort is to segregate ethnicities as a means of dividing and conquering the populace.