Well if you believe Wikipedia: https://en.wikipedia.org/wiki/List_of_tallest_buildings_in_N... the tallest NY building voluntarily demolished to date was the Singer Building at 185m. My guess is that there is a point of diminishing returns at which there is no economic return in knocking down your existing building to build an even larger one. In JPMorgans case they have a 50 year old building that they are planning to replace with one that doubles their office space so they seem to have a good rationale.