Which is where your argument falls apart. Cities aren't closed systems. The price reductions won't trickle down to the bottom of the market because at some point there is always going to be people who are willing to move in from neighboring neighborhoods, cities, or regions. It isn't like the only people who live in San Francisco are the only people who can afford it. There are plenty of people who could afford the worst possible home in San Francisco but decide not to move their because they can't afford a home they want. Once those middle of the market prices come down, people will move in to fill that new supply.