Holland is the densest nation in Europe, but still has substantial farmland, so the urban parts are very dense. It is flat with a mild climate, and the entire urban areas combined would fit into an area smaller than New York City. This is somehow a reproducible model in your mind.

Then there are real economic and social trade offs that need to be seriously discussed. Historically the beautiful, walkable, lovely human scaled cities we all love before the automobile had large inequalities in land ownership. A small number of landlords owned most of the living units and everyone else rented from them. This was because the jobs were in the cities, so landlords could effectively tax people for having access to the high paying jobs. Once they got 1/3 of everyone's income, they could roll that over to purchasing more units faster than the renters could both pay rent and save to buy their own house. This creates a situation where a small number of families own most of the beautiful historical buildings that renters occupy in those lovely downtown cities.

It was the introduction of the car that created a real middle class and increased home ownership allowing people to escape the thumb of their landlords and buy cheaper land surrounding the cities while keeping their jobs in the city. This shifted the economic rents captured by urban landlords and transferred it to auto manufacturers and other producers, shifting resources away from rent extraction and towards production, which spurred economic growth, home ownership, and family formation all together. The growth of suburbs was merely a byproduct of this economic geography.

As transportation technology improves, cities become less dense over time. All cities, the world over, are slowly getting less dense. Living units are growing a few percent each year size, so what used to house a family over time becomes a studio apartment, and in this way, even as the buildings remain the same, urban density declines. Even builder taller buildings doesn't overcome the inexorable decline of density, a process uninterrupted since the early 20th century.

Global populations are diffusing, spread out constantly trying to stay one step ahead of the urban landlords and take better advantage of new transportation to buy units farther away where land is cheap enough for them to buy their own small lot.

When you start reversing this, you also start rolling back homeownership rates. Those lovely, dense walkable cities in the US are majority renter populations. They are also demographic sinks -- people don't reproduce at anywhere near replacement levels compared to more rural environments where homeownership rates are higher as are fertility rates.

In other words, there are real inequality and social sustainability problems with dense living. It's not even something that is demographically sustainable. These cities require a constant inflow of people from more rural areas or immigrants from other countries to prevent demographic collapse.

That doesn't mean that there aren't solutions -- perhaps there are! You can adopt public policies that disincentivize renting and subsidize homeownership in cities. You may (or may not) address the demographic collapse of urban areas. Spain (and the Netherlands) have massive problems with demographic collapse but have achieved high home ownership rates, but it addressing transportation promotion with homeownership and family formation all together, because all of these are intimately related.

This is something that the anti-car crowd has never done, nor is it even on their radar.

That people leave the city because they want to buy their own house and have kids, and once they are in the suburbs you really need a car -- that doesn't seem to register. Instead, it's all talk about building more infrastructure and this will suddenly reverse these trends. This isn't a serious engagement of the issues.