The price of land is much lower if the entrepreneur goes big and build a whole new town from scratch. There are however some challenges with this approach. None the less, it's being done all the time in Asia.
Engineers generally like big problems like this, and a world where Global Warming will displace large populations needs to get really good at managing huge engineering projects.
Just look at Russia and their Crimea bridge, St. Petersburg bypass bridges, Vladivostok bridge, infrastructure projects associated with the Sochi Olympics, the Universiade games in Kazan and so on. And now Trump says that he plans on fixing the USA's decrepit infrastructure that has been neglected for the past two generations.
Engineering solves problems.
China building unoccupied cities has more to do with the fact that the average Chinese citizen is not allowed to speculate or invest in much other than real-estate.
They failed in that there was a huge investment to build them, but the expectations for these cities were never met. Additionally, there is tremendous upkeep or maintenance.
For example, those new roads are crumbling and require extensive repair work. Think of a road that is 5 lanes wide, but a 1 lane would suffice to serve all of the traffic. The unused portion will still degrade, even though it's not used.
It's kinda like buying a huge house before having children. You still need to fix, maintain and clean the extra square footage, even though it's not used.