We have had agriculture for quite a few centuries too, but that doesn't make modern agriculture sustainable. Contemplate your 'sustainable' city without the impoundment of rivers to supply water to produce electricity or the flooding of vast areas of the countryside to supply water to wash the dog shit off the pavement into the waterways to pollute the harbours and beaches. The problem with city folk is that when most of them turn on a tap they have not the faintest idea where the water comes from, they switch on acres of neon without the slightest regard for where the power comes from and they flush their toilets without a second thought to where the shit is headed. I guess if you ignore these things, as most city folk do, then perhaps a city is sustainable. But only in your imagination.
Cities are for the dogs.