[mikraig]

> I don't think we should be striving to turn the entire Earth into Kowloon.

Oh of course not, that's an absolutely extreme example.

Yes, advanced societies tend to have a decrease in birth rates. Yet, they still keep getting denser, as people move to cities. Density, if done thoughtfully, can lead to increased economic output, better health, better environmental outcomes, less traffic (holding population constant, anyways), etc.