"But we shouldn't ignore the achievements of an empire that managed to feed 22 million people on 15th-century maize farming technology (in some ways less advanced than elsewhere, and almost without domesticated animals)."
Why not? There is NOTHING to emulate there. If we needed their farming technology (we don't), sure, we could copy that, but the real lesson of the Spanish and the Aztecs is that thank God, good triumphed over evil and the world was made a better place. Somehow, that lesson has been revised into a lie, that acquisitive Europeans and domineering and oppressive Christianity conquered an admirable people, stole their land, and destroyed their civilization. That is untrue. The fall of the Aztec civilization liberated the good people in that society and vanquished their oppressors. It was a GOOD THING.
Much earlier, in the times Punic Wars between the Roman Empire and the Phoenicians, there was a similar situation. The Phoenicians had a sophisticated trading empire, a navy, and many technological advantages, but the Romans stomped them into the ground, killed everyone, and salted the earth. You see, they worshipped Moloch, an evil god, and their primary religious ritual was tossing live human babies into furnaces as sacrifices to their god. (Hmmmm, this is really making me wonder about that theory that all religious are really equal and really worshipping the one God.) Carthago Delenda Est (Carthage must be destroyed.) It was. Good Riddance.
There is never a good reason for whitewashing history.