Could those houses be re-built almost anywhere else?
Is there some value to living below sea level that justifies the immense expense required to rebuild in this area?
While there was a city there, there was a lot of sunk costs that made it worthwhile to protect the property that had already been built. But now that is no longer the case. These areas are destroyed. There is simply no reason to spend hundreds of billions of dollars to put them back, when they will just be destroyed again, down the line.
Again some areas will not be redeveloped. There is a plan being formulated down there. Its politically hot but its proceeding