You are absolutely right in this. Genghis Khan was not only a conqueror, but also an efficient administrator. The various khanates across central asia were a result of mongol invasion. In most of the ancient world, conquest also meant integration with local population. It is not an superior race ruling over inferiors.
On the other hand, what knowledge did european conqueror save? We have all the lost civilization, primarily because europeans did not care to save anything that is not christian.