Maybe it’s because people are getting richer
Local progressives inaccurately claim that the South Bay's middle class dwindles because rich techies are taking all the money and forcing everybody out. The claim is false on the facts and shows a limited understanding of economic principles.
Here are the facts: The U.S. Census Bureau suggests that in 1967, 8.1% of the U.S. made more than $100k (that's in 2016 dollars). Fifty-three percent made $35-100k. Thirty-eight percent made less than $35k.
Fast forward fifty years to 2016 and see the rising tide at work. Here are the percentages in the same categories, and in the same 2016 dollars. Twenty-seven percent of the population makes more than $100k--that's an increase to this top bracket of 19%. Forty-two percent are in the middle zone of $35-100k--that's down 11%. And the bottom bracket counts for 30%, down 8%.
And this data does not even take into account the value of redistributive government programs to people in the lower brackets. The conclusion: the data unambiguously shows that Americans are getting wealthier, and the decrease in the middle class is not because people are getting poorer, it's because they're getting richer.