Well, is it nonsustainable today because of people being modern?
Mexico City has the worst pollution in our hemisphere. Being located at such a high elevation and surrounded by mountains (a common problem in human development throughout the Central and South America's), it traps all the pollution in their valley.
There is no water, but thanks to modernity they can pump water to them, at the expense of other valleys around them no longer having water and turning farmland into dry wastelands.
Transport is abysmal to get to their high elevation, and routes in and out are very limited.
Modernity has given the city time... but at what cost?