That's a good point. It's been a good place to "raise kids"--if your kids are little companies because there are lots of great people to work with, impedance is low because of shared norms, and outsiders are aware of the importance of this symbiosis. Not unlike Hollywood.
Like Hollywood, Silicon Valley is subject to "runaway production" in which some other entity provides incentives to a company/production in the hopes of jump-starting a similar situation in their own locale. This provides some incentive to Silicon Valley/Hollywood to not milk their own cow too hard or too often.
When a large company relocates, there's lots of news and hand-wringing, but really, big companies are all grown-up and ready to leave home: "Enjoy North Carolina, son, we'll see you in Cupertino at the holidays!" For startups and indie productions, the environment has to be right. The long-term risk to California is that it no longer provides the confluence of people, ideas, economic environment, regulatory/policy environment, etc., that spawned Hollywood, Silicon Valley, biotech beach, the aerospace corridor, etc.
In my experience, Hollywood and Silicon Valley are unique environments that are valued (by outsiders) for their contribution to the tax base, ability to create jobs, etc.; i.e., instrumentally. But seeing them only in this way will lead to their destruction, not unlike old-school strip-miners who pillaged the hillsides and then bailed. Outsiders cannot be expected to understand, so it is the responsibility of those who exist in and benefit from unique, complex, symbiotic communities like silicon valley, to steward the community and protect them from predators who see fodder for their own will to power.
Well, a large software company has varying needs, some of which may not be met well in the Valley. For example, Zappos started in SF, but moved to Las Vegas for the lower cost of living.
That depends on what incentives you could wring out of the state and local governments.
If you're a large company, you were once a small company and there's been no place better than silicon valley to grow-up as a young company. I think this is changing.
