I disagree about the dynasties. The Norman-Plantagenet line lasted a little over 400 years. The Capetian-Valois in France lasted nearly 600. The Hapsburgs lasted right down to the 20th century, as did the Romanovs (almost 400 years for them). In China the Han endured for over 400 years (with one interruption). And the Japanese imperial house dates back to a time before the Japanese had written records
The Roman and Byzantine empires were uniquely unstable due to a failure to have a clear rule of succession.
I also partly disagree on the matter of ethnic diversity. Yes, I would certainly expect more linguistic diversity — a lot more. But we should note here that the Americas before 1492 had just three population types (considered genetically), and one of those was confined to the Arctic, the other in fairly limited territories in the Northwest and desert Southwest of North America. The people of South America were all fairly closely related, apparently all descendants of the same founder group, despite the considerable linguistic diversity.