Hahah...  perhaps you better follow your own advice and have a word with the Aztecs as they were never an "empire" in the total-itarian sense that is used, and implied, today; they were rather a loose coalition of city-states more engaged with puppeteering their neighbours/vassals than razing everything to the ground.

Your way of reasoning is leading to nowhere because you extrapolate the (supposed) objectives of today's imperial doctrine to the past. Prior the industrial era, however, things were different: for starters there was never "surplus population", on contrary human resources were often scarce and valuable; and back then natural resources were abundant. What was valuable were manufactured goods as they involved (i)human labour; (ii)human expertise; and (iii)frequently long haul transportation. That is one of the reasons why successful empires of the old type (like the Ottomans) build a space essentially free of sharp antagonisms where normal life could continue with productivity and trade in relative peace.

I am yet to read the book that prompted this thread, but from reading the excepts and others' commentaries the same mindset apply to the "Mongols". While certainly violent they were focused on controlling, benefiting from, and advancing existing arrangements; conquered entities usually continued their existence more or less unchanged. Compare and contrast with the conquest of the Americas which was by all accounts the proto total war: populace dispossessed; almost completely exterminated; indigenous culture razed to the ground; almost nothing survived from the natives' way of life.