Certainly no city is recession-proof, but Detroit _was_ almost totally invested in a single economic industry: car manufacturing.

Silicon Valley's emphasis - digital computing devices and software - fuels multiple economic sectors. It seems far more diversified than the auto industry.

But tech can also pick up and leave much faster and required less reason to stay clustered (factories, transporting materials, supply chain etc). Add in the fact the world in general turns faster, I don't think SV can keep the tech industry for how long Auto Manufacturing remained in the Mid-West.

Doesn't SV also have a significant chunk of the biotech industry? Even if software departs doesn't mean other forms of high-tech, with talent and research funneled in by Stanford, Berkeley, etc. will follow suit.