The thing is businesses can not rebuild and move buildings quickly. When jobs disappear rents will go down. It seems impossible San Francisco could be the next Detroit but it’s actually not that improbable. There is just so much inertia keeping businesses from being able to move that they can’t or won’t. The actually physical cost to spent billions to build a headquarters but also the need to be closely located to other businesses that you will work with. Without this inertia you would see much more businesses leaving than are now, and there are more than ever leaving.