Like the referred to black-hole, high-density cities too are energy sinks. Take fossil fuels out of the equation and let's see what remains....

Spoiler: not much...as even the act of having a glass of clean water simply becomes unaffordable for most.

Human density is a causation of the density of the cheaply available energy. Change the density, change the spatial relationships. It's all rather straight forward. Prime real estate today in NY will then be worth nothing. (perhaps the copper wiring as scrap metal)

If we want to understand how a (more) bio-based complex society would look like, the Maya civilisation managed to maintain their medium density, green (because food producing), cities sustainably for over 2000 years. (but won't be repeated there because of rising wet-bulb temperature in the Gulf of Mexico region).