> lead to Dredd-like Mega Cities just taking over the entire continent

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.