Should We Expand Manhattan?

Let me correct that for you: Build hyper expensive housing for 250k rich people to invest in and hold empty or rent out as air BNBs, to act as a sparsely populated sacrificial landmass to absorb storm surge so it won't reach the trading floors which technology is in the process of obsoleting.
Another idea: distribute population and economic activity around the nation's many mid sized cities  where housing is affordable for actual residents, who won't have to live on garbage and old concrete dumped into the sea.  How about that, Mr. economics professor?