Knock down three adjacent homes and you can get maybe 6-8 levels, which would be no more than 40 families.
The problem is introducing the zero-setback designs organically - as the people there likely receive little to no benefit, and the people who do receive the benefit aren't there now.
E.g., a house could easily be fit between mine and the neighbors, but we wouldn't really benefit from the density improvements, and the family that would move into the "missing middle" doesn't currently exist here.
I'd love if our lot was zero setback, as I'd build an addition right up to the property line instead of having to try to find another lot/house in the area we want.
Infinite cities only yh happen with infinite population.
(others have already pointed out that populations are on track to fall shortly and there is no reason to think that trend will reverse though nobody knows)
This is impossible. Look at even the densest cities today such as Hong Kong, with many 50-storey buildings packed closely. HK as a whole has maybe 25% land area allocated for buildings and the rest is forest and green space.
Or consider Tokyo - sure, it is a big sprawling metropolis and pretty much an uninterrupted patch of concrete. But the urban area does eventually end, and much of the land area of Japan is mountains, forests, farms, etc.