Detroit was once one of America's finest cities, basically the embodiment of the American dream. Once the economy that floated all that ended, it ended too.
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.