History shows us empires never last.
People groups do. Israel, for example, has been around as a distinguishable national and genetic entity for something very close to four thousand years notwithstanding its lack of a country to call home for most of that time; the Chinese have been almost completely ethnically homogenous even longer. In this respect at least, China holds a tremendous advantage in our present day should the US opt to go to war with them over Taiwan. Ideologically divided right down the middle, some say the US is on the brink of civil war; though occasionally experiencing internal unrest, the Chinese are comparatively unified.
But that’s the problem with multi-ethnic empires: they don’t have the kind of cohesion and staying power that kinship produces. The modus operandi of empire building is perpetual expansion and absorption of new people groups, whether through conquest or immigration. At some point, those so absorbed inevitably begin to influence the empire in favor of their own ethnicity and agendas.
Historically speaking, diversity is the opposite of strength. Biblically too.