There definitely is a limited number of millionaires that an economy/population can support. Also, comparing across time is rather deceptive. The north korean military is much more powerfully compared to khan or alexander's armies. The comparison is pointless considering the north korean military isn't traveling back in time to fight the mongols or the greeks. Saying middle class americans are rich compared to romans, persians or carolingians is just as meaningless. You could argue that a homeless man with a smartphone is "richer" than napoleon since the homeless man has access to more computing power, better medicine, better everything. But that's meaningless and no solace to the homeless man since he isn't traveling back to the early 1800s with current technology in his pocket. If we follow your logic, a north korean starving in a political camp is wealthier than Caesar. I guess that could be true, but meaningless and empty.

It's also a deceptive tactic people use to compare apples to oranges. We have to compare today with today and even more importantly, we have to frame money within the zero-sum context of power. Money and power are interchangeable and viewed in this context, wealth is a zero sum game.