1. That wasn't the original claim, which was "There should be enough housing for everyone that wants to live in a given area". People don't want to live in Manhattan 2, they want to live in Manhattan.

2. "Build more Manhattans" is easier said than done. Even petro-states with astronomical amount of money to splash around have a hard time building a "Manhattan". Dubai might be very shiny and the financial capital of the middle east, but it's no Manhattan.