That's a bit complex for an HN reply but anyway in general the key of density is "not too much, not too little". Modern cities are in the "way too much" level, some countrysides in the "way too low" level, Rivieras are the today mean IMVHO. Witch means enough dense to have a certain economy of scale, a certain sustainability of services, but not too much so they are able to evolve.

Let's take the example you do on "an apartment need less energy" because being stacked ..., that's true, in theory. In practice, at least here in UE, most apartments tend to be mean-old buildings, grown up quickly in some economic boom where energy costs was not something anyone care. You can't insulate effectively such buildings, you can do something at a high price for little results. While single-family homes are mixed, some are old, some are new and almost ALL can be rebuilt without absurd issues. Tall buildings can't: they are too dense, they are build in places where it's not allowed to build for good reasons (like some social housing of the '60s build one after another on a steep hill and a single crappy road to access all etc). So essentially they can only remain in they sorry state until they collapse or get demolished anyway.

How much they have cost in structures? Because a light 2 flat home it's a thing, a 12+ flat building it's another. How much big settlements have altered local climate to the point that any cities have it's own proper microbial signature, natural water system alterations etc? Spread settlements source water locally, so they do alter a bit, but not at a city scale, they pollute locally, but again not at a city scale. An example I found is http://www.newgeography.com/content/006840-high-density-and-... but you do not need real measurement it suffice to think how nature work and how humans can or can't integrate with it.

In a spread Riviera you still get a doctor nearby, because the density is enough, but you only get small buildings intermixed with nature. Yes, you need to travel, but you do not need expensive and invasive road infra, bridges, tunnels, metro etc just simple roads with some animal passages here and there, the soil is still permeable almost everywhere and so on.

In evolutionary terms: classic deliveries from on-line retail? No issues: just replace your mailbox with a small package home and that's done. Drones delivery? No issues. There is plenty of space. Just change the package delivery to offer an aerial access. Future flying cars? No issues. Just sacrifice a small area of your garden to create the lending place. You have room for p.v., to store water, ... since you can be A BIT autonomous any "service issues" from electricity grid to water supply it's not so urgent to fix and so on.

Long story short: economy of scale works when PRODUCER are nearby, since now in cities almost no one produce anything, there are just services and resellers in cities there is no more an economy of scale. Since WFH have issues but works and issues can be solved well, or at least mitigated with a bit of time, most westerns cities who are tertiary sector workers place have anyway no reasons to exists anymore. Transition it's not cheap, but in ALL CASES will happen because as I said before cities can't evolve and we evolve. As the time passes buildings reach their EOLs and rebuild them it's next to impossible or simply an absurd effort. There will be and already there is no room to move anymore, tied to crappy public transportation systems who are not sustainable if more effective and so on.

If we try at a slow peace it will hurt much less but try to deny that stating it's possible to keep building cities "but modern one", modern today, obsolete tomorrow, with the same issue is as hypocritical as my friend...