It's not a numbers thing. NYC is the financial center of the planet and a huge percentage of our major corporations are headquartered there.
The British burned Washington DC and somehow we managed to survive. Only a small percentage of U.S. commerce is dependent on major corporations -- about half is small business. If the U.S. headquarters of Wal-Mart is destroyed, the local Wal-Marts will continue to operate. The local Midas Muffler shops will continue to operate, etc.
One would hope that the lessons of the World Trade Center were taken to heart. Concentration of vital records, services and communications is dumb dumb dumb.