But how many people and corporations would be willing to take that risk? Sure, it would be a lot cheaper, but Detroit simply doesn't have the resources and infrastructure to support the same things as San Francisco. Unless there was a huge crash in silicon valley that affected companies there the way the automobile industry collapse affected Detroit, there's little incentive to leave. In their eyes, don't fix what ain't broke.
Kansas City is on the verge of a tech boom due to Google Fiber and cheap costs of living. Chatanooga, TN is starting to bloom as well; you can get affordable gigabit internet, buy a house 5 minutes from downtown for under $100K and are close enough to high tech areas for work while you slave on your game/apps/site/idea. Why in the hell, with options like that, are you wasting your start up capital on $5000/month rents for apartments in San Fran?