Simon: To occupy and transform a country and introduce your culture and values you need to be there for a long time so a generation grows up accepting them as the nom. That means doing all the boring stuff - run bureaucracies and infrastructure.
That's the lesson history teaches us, so I agree with that conclusion. When Alexander the Great conquered the Iranian (Persian) Achaemenid Empire, Hellenism (Greek influence) gradually became the norm, albeit with local characterisitics as in hellenised Bactria (Afghanistan) and hellenised Judaism in Palestine.
in contrast the Mongolian empire did not endure long enough to have a great and lasting influence, except possibly in some regions now part of modern Russia.