Empires stay empires in the original sense only when there is a core where people are privileged, and colonies where people are exploited and lack rights. Roman Empire stayed like that for a long time (even Latin rights were introduced relatively late), Russian Empire had a lot of that, and British Empire was practically an epitome of that.

I don't see how the modern US is comparable: equality is formally enshrined in the Constitution, and whatever inequality exists is delineated more by ethnic / cultural lines than by geography of birth or living, driven by private xenophobia, not institutions.