At that point you need to put your heat exchanger pipes underground, which is more annoying than air-based AC but not insurmountable. The temperature 20 feet underground will be about 50 F year-round. Of course, if a city of millions tried to do this all at once I have no idea how the local ground temperature would react.
> 20 feet underground will be about 50 F year-round
Is that the case in India? I was under the impression that while underground temperature is consistent across seasons, it does vary considerably across climates, with warmer climates generally having higher underground temperatures. For example, while much of the central US has underground temperatures around 50F, southern Florida, California and Arizona have areas with temperatures in the upper-70s[1].
I can believe the ground temperature is higher nearer to the equator. At the same time, you don't need the ground temperature to be very low, just low enough for the heat exchanger to function. Alternatively, you can dig deeper until you hit a lower temperature.