[realreality]

Cities have never been and will never be sustainable. You can cram 100 people into an apartment building, but you’re never going to be able to grow all of their food in a rooftop garden.

Although high density can be more energy efficient, it’s not the only metric to consider. Apartment dwellers are completely dependent on importing resources, while rural people at least have a chance at breaking the dependency.

[immibis]

You're saying that cities don't have enough surface area to collect the energy for their residents, but that's irrelevant since the energy doesn't have to be collected within the city. There's nothing unsustainable about a dense city core surrounded by fields.

At best, you've proved that a planet composed entirely of cities is unsustainable. I hope nobody wants that, but sadly some people do.

[realreality]

It's relevant where the resources come from. If a city depends on importing from nearby or distant fields (as well as distant mines and factories), what happens if the people who live in those places do not want their land exploited for the benefit of city dwellers? That is why civilization is based on violence: https://derrickjensen.org/endgame/premises/