Monolith "megacities" may become practical, and even necessary, in interplanetary colonization:
We may not be able to simply build anywhere we wish on a given planet, and the horizontal sprawl of "traditional" cities in unexplored and potentially hostile environments would be risky.
Having a single place for spaceships/landers to dock at/orbit over, right next to the facilities for refueling, cargo transfer, storage, processing, distribution etc. would be desirable, so more-or-less vertical megastructures may be more practical there.
We may not be able to simply build anywhere we wish on a given planet, and the horizontal sprawl of "traditional" cities in unexplored and potentially hostile environments would be risky.
Having a single place for spaceships/landers to dock at/orbit over, right next to the facilities for refueling, cargo transfer, storage, processing, distribution etc. would be desirable, so more-or-less vertical megastructures may be more practical there.