The first sentence I'll agree with, but I'm not sure your second stands up to scrutiny. The Empire of the Medes and the Persians, the Roman Empire, the Russian Empire at its height, and within your time scale, the British Empire and the Soviet Union, were all ethnically diverse. (Heck, the dictators of the Soviet Union weren't all Russians, Stalin was Georgian -- a fact folks in Ukraine like to forget when remembering the Holodomor -- and Khrushchev was a Ukrainian. Lenin used Latvians as shock troops during the Russian Civil War.) The monoethnic Axis powers got crushed by the ethnically diverse United States, British Empire and Soviet Union.
My definition of diversity appears different than yours.
Diversity as what we presently view is the
forced incorporation of different languages, ethnicities, sexes, and inclinations, rather than the incorporation of the most competent individuals regardless of these attributes. It forces acceptance of multitudes of divergent ways regardless of their prominence, productive strength, and goal orientation. It is achieving variety itself as a goal rather than having productive outcomes as a goal.
In the majority of cases you cited, there are always groups and individuals who achieve success who are outside the chief ethnicity of that country; however, they were not forced into these roles, but achieved them in unison with the prevalent national attitudes.
As an example, the German nobility who assumed the English throne did not make England into Germany but assimilated themselves into the British Empire. Languages were not changed in Britain either from Germans running the country.
It is the assimilation which is important, and the current diversity attempts directly interferes with assimilation. China understands this well.
Diversity is creating a Tower of Babel.
