"Dense" cities are necessary for achieving the efficiencies of density, but "hyperdense" cities are not. You can have a city where all residential buildings are five- to six-stories, with ample green spaces and every street lined with trees; that would be a dense city, despite being less dense than Manhattan.

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).