Again, not true. That employee can simply... move. You've ignored that.
Well, yes, but we're arguing that they have the same leverage in their new place of residence as they did before the move. If they have to move back in order to exercise that leverage, their leverage is actually location-based, and they will have to choose between their old salary and old location and new salary and new location.
These companies are used to this sort of thing. Relocation costs are nothing compared to the cost of hiring. There is effectively no friction on the company's part. Yes, the employee needs to be willing to go through the process of moving, but, provided they're willing to pay those costs, their leverage in the situation is unchanged.
Yes, there is a subset of people that have disproportionate leverage no matter where they are at (by virtue of being really really good devs). But that represents only a minority of SF devs.
A family of four enjoying a large house in some suburb for a reasonable price may find the idea of living in a 2br apartment in SF, or paying a truly exorbitant price for a single family home in the bay area, unappealing.