> some people like being around other

Liking does not means is cheap or sustainable though.

> few dense core cities are better than endless suburban sprawl because by taking up less per person, there's more left over for nature.

Actually it's the opposite, first because you do not need less per person, you do need more in natural resources:

- big buildings demand much more infra around and themselves consume much more raw materials to be built, beware that OLD buildings are another beast respect current one with seismic and fire safety, energetic performances, ventilation and so on. In the past a small 4-5-storey building (let's say 20 apartments) was on-par or even a bit cheaper than 20 correspondent homes because homes tend to be bigger and have 20 roofs. But such ancient buildings does not care about how much energy they need to heat, they have no cooling, no seismic designs, ... they are just "CA boxes" stacked around with small additions (clean and dark water, electricity, windows etc). Things have changed, elevators, stairs, also have their slice;

- supporting infra is much more impacting as density, few dense are is like sit on few staple, it hurt, MUCH, you create heavy masses (subsidence), divert much water altering water cycle for a very larger area, produce much concentrated pollution, absorb much heat (a big slice of thermal mass exposed to the Sun) and so on, again modern cities are not old ones still made of small buildings where peoples tend to work at ground floor living "just upstairs", there are still no factories in cities anymore, because nowadays change is quick enough and scale vary enough we need room to change and a dense city is too dense for that, so you still need to go back and forth the city and moving all goods etc;

- finally the most important: cities can't evolve. Today "A-class" homes are tomorrow G-class, today needed infra are tomorrow relic and missing new needed infra, just see how hard is in dense EU cities parcel delivery. We have had parcels before, they were just rare and letters just common, now it's the opposite so they can't got delivery on feet by the postman nor they enter the small set of post box in the hall and it's a new logistic nightmare, such a small change such a big issue. To change a city you need to rebuilt it and that's dramatically expensive.

Also, nowadays in spread area (beware, spread A BIT, not large ranches around the Steppe) you do need to travel MUCH LESS than before, first all works doable from home should and can be WFH, witch just erase commuting for a big slice of population, having longer trips to buy food means you stock more at home and you have room to stock, so for instance instead of buy small packaged stuff where the package is single use and prominent respect of what's packaged you buy large stuff with much less disposable package. Being able to do much more via web means again much less trips. In the end you travel of course, but much less than before, and you can travel electric on scale, because most people can recharge at home or at work (where the work is in a fixed place, from local p.v.). You probably thing about USA style suburbs where there are ONLY homes for a very big area and somewhere else ONLY shops etc, such design it's not the spread design, it's the Ford design to sell more cars. The kind of density I advocate is the EU Riviera model, where homes and shops are intermixed, so you still have few shops in 1 km, a school nearby and so on. Where you can live in a 30km radius instead of 15' walk ignoring the immense infra and supply chain needed to serve you in the 15' setup.

The only who benefit from modern dense cities are those of the financial capitalism that need such "big scale" to rule, at a less dense scale they fail. Maybe local foundries are untenable but that's not true for much more, in the end we can have few cities to serve some productions composed only of workers for a certain period of time, of course workers have family, children but it's still a temporary location, they'll go out when retiring or changing job and in that case such cities became much smaller. The New Urban Agenda essentially agree we can't build large dense cities, Neom, Arkadag, are good examples of such failure as original Fordlandia, but they still hope to build small cities of essentially slaves, who depend by third party services, owning nothing, to live, and even if that's might be possible it's not good for nature nor for us. Now take a look at "urban" air mobility push https://www.easa.europa.eu/sites/default/files/dfu/uam-full-... did you think it could be "urban"? I think it PERFECTLY much a spread living, matching actually the old model of the gentleman who run by car seeing slaves working by feet around, the advertise like https://appliedevtolconcepts.com/ who perfectly mach '30s car advertisement.