What city are you imagining where ten million apartments would be built with no demand for them? I'm assuming the city with ten million apartments and half a million residents is economically depressed, since that many apartments were clearly built for a much larger number of people.

Here's a better question: under a realistic model of building, where building slows down when landlords and property owners feel they have maximized the profit they will get out of it, by what date do you think housing prices will be low enough that current residents who rely on rent control to stay in SF will be able to live there again? 2025? 2035? 2050? What is your recommendation for those people in the meantime?