Right now, space is criminally underused in cities or allocated so inefficiently that we don't really need acrologies yet. We can get more green in cities and making these places more pleasant and human space to live.
I don't foresee a future where any city feels the desire to model itself after the Kowloon Walled City in terms of density, because in order for that to happen it would have to imply that physical space itself is the bottleneck for the population, rather than things like the availability of energy/food/water (which was true for the KWC for historical/political reasons).
So, essentially Berlin inside the ring. Each "kiez" (a neighbourhood) is sort of a village of its own.
I lived in Hong Kong just under 10 years, and a friend of a friend grew up in the walled city.
As far as I understand this happened even before the handover.
The more interesting question is why so many people feel this place in the first place. Why this puzzling fascination? I don't have this with any other place.
Why do the pictures feel like home?
While I shouldn't, I remember it as people struggling to make ends meet but also as an incredibly happy place because of the tightly knitted social fabric. The need to help others and the need to be helped creates top notch relationships.