Silicon Valley wasn't called Silicon Valley because it had lots of sand. It had the whole supply chain before it upped out to Asia. Even today, china does not produce many of the higher end components that go into say an iPhone, not as much as Korea, Taiwan, and Japan. The supply chain still depends on lots of imports, and none of that is coming to china anytime soon (or it would already be there, like high end fabs).
i can grant you all of that, but i still have little idea how long it would take to resurrect the parts of the supply chain which have been moved to China (and left to rot in various places outside of China).
maybe it's better to focus the question on a single region: how long would it take to resurrect the manufacturing component of SV's economy to the level it was at 25 years ago?
a year or two? five years? a decade? what would the time scale be?
If you consider the Ipad came out in 2011, which was 5 years ago (for a few more days), I would say 5 years. Touchscreen devices make up about half of the global electronics industry, so by extension half of Shenzhen was built to accommodate that.
With a focused agenda, the US could rebuild a supply chain in 5 years if it wanted to. I think it is unlikely to want to however.
The question is really moot as long as china is willing to do the dirty work for us at a discount (actually ruining their environment in the process), there is just no incentive to regrow that capacity. No one in the west will say no to free money.
But as long as the need and incentives were there, it would be rebuilt pretty quickly, possibly in radically different and improved ways (more automation, 3D printing, upgraded processes....).
