You can't just tear down some 4-story building and put up sky scrappers. There is a maximum density capacity that a lot of Manhattan already exceeds. The trains, sewers and electrical requirements need to be thought of too. Especially the trains which are already way over capacity.

Look at what has happened in Williamsburg and the L-train. As more and more warehouses have been converted into apartments and new condo's have gone up, the L-train has become awful for large parts of the day. And the train runs as often as possible during those times.

I'm not saying you can't do these things but I do think other things need to be considered.