This real estate hyperinflation is going to destroy the very engine of class mobility and youthful enthusiasm for a better life that powers the Valley. When that's gone, the Valley will decline. When the Valley declines, the local real estate bubble will pop.
Want a historical example? Detroit. It once had the highest average income in the U.S., and now look at the property values there. The reasons for its collapse are different and it's unlikely that SF (which is desirable for other reasons like climate and landscape) would ever fall as far or as fast as Detroit, but it's illustrative nonetheless. It shows that one should be cautious about long-term value in overinflated one-horse-town economies.
And if SF is not a one-horse-town, it soon will be. Industries other than tech will succumb to the lure of lower costs of doing business and leave.