Eh, wot? A 'metropolis' is only 25,000 adults per DMG, plenty of the cities in Greyhawk have that number or higher, in fact I think most of the national capitals do, and Greyhawk is about the most medieval-y, least urban, of the main campaign settings! In something more like the classical world/Nehwon/Young Kingdoms model common to swords & sorcery, of which my own campaign world is typical, you're looking at far more 'metropolises', with total populations up to around 1 million (Imperial Rome, Byzantium at its peak, Peking).
And remember you're rolling 4 times per class per metropolis! Per DMG RAW each 25,000+ city has four Wizard of level 12+1d4, 4 Fighters of level 12+1d8 (& 8 of half that), 4 Druids, etc etc etc. You will have a good few level 16+ NPCs in every one.
This is way too much even for Gygaxian Greyhawk IMO, never mind Wilderlands or Nehwon. For one thing it renders conventional armies useless - maybe the kingdom can muster 10,000 Warriors, but they're only good for peacekeeping, all the real fighting will be done by the dozens of level 10-20 superheroes. You get something more like 'Way of the Exploding Fist' than LoTR.
So, I change the demographics. No biggy.