We have the internet and cheap transportation now, so people (with resources) can more easily move to where they want to live.
I can conceive a future where even if tech were somehow displaced, then other people with money would come to silicon valley, simply because they want to and they're not tied to another location, like richer people may have been during Detroit's heyday.
I relocated to Colorado a decade ago, swallowing the cost of living salary decreases, but I'm coming out better in terms of quality of life. If California, for a split second, achieved even close to the affordability for quality of life as CO, I would move back.
If Detroit and San Francisco were equivelant cities (culturally, cost of living, etc.) I can't see why anyone would pick Detroit.
A better comparison to southern California would be the Iberian peninsula. Spain and Portugal have pleasant weather and are well connected to the industrial heart of Europe.
Many want to live in these countries but most can't make a living there.
What if the secular demand for living in Silicon Valley is higher than Detroit? What if Detroit was desirable due to the economics and infrastructure constraints of that time, but what if people like the other aspects, such as location and weather,of living somewhere?
Not that it’s not possible for that to change either, but my point is different pieces of land may be more or less desirable than others outside of economic conditions.