There are other possible scenarios for a housing market plunge in SF. For example, if California tries to sharply raise taxes to help pay for their huge government infrastructure, tech companies may decide that they don't want to be in California anymore. They're already hedging their bets. For example, Google has a sizable presence in NYC. When the first big tech company announces that they'll be moving out of Silicon Valley, the housing market will take a dive.
And it's not impossible to run a big tech company outside of Silicon Valley. Microsoft and Amazon seem to be doing OK in the Seattle area. The higher housing prices go in San Francisco, the easier it will be to recruit people to go elsewhere.