These are my personal observations: When I first moved to the SF Bay area, I was surprised by the amount of visible homelessness and by the number of service economy workers who drove nearly two hours to and from work. I also had my first experiences with the "campuses" of major technology companies, where money was made hand-over-fist. I think the recent widening income gap is a natural consequence of the technology…
Information, professional, and business services collectively account for less than 25% of Bay Area workers. At least 75% of the people currently affording lives here are not in tech. There’s a big difference in what happens in the housing units that are up for grabs vs. the city as a whole. http://www.vitalsigns.mtc.ca.gov/jobs-industry
And it is the growing industries that drive the market rate for housing. Someone who bought in to the market via purchase or rent-regulated lease five or ten years ago has a significantly lower cost of living than someone entering the market today.
Exempli gratia: (in California) If you're living in a $2 million house that you bought for $400 thousand, you might not be able to afford to sell and buy something else at a similar price due to the tax code.
Exempli gratia: If you're living in a rent-regulated apartment that's now leased 40% under market value, if you lose your lease for any reason, you may not be able to afford the market rate. And your landlord has greater incentive to get you out of the unit.
Thus the well-tenured school teacher making $100 thousand per annum[0] can afford to live comfortably, but when this teacher retires, a qualified replacement offered a salary of $66 thousand[1] may choose to live and work elsewhere.
[0]http://www.sfusd.edu/en/assets/sfusd-staff/contract%20and%20...
[1]ibid