That's just it. California can't pull up needed resources because the state can't survive on silicon valley alone and they aren't likely to ever do the smart thing and start a desalination plant to get a useful amount of water. The Colorado River supplies most of the water in the southern part of the state so the southland and surrounds will have to renegotiate their water deal with what they consider a hostile nation.
Also, they can kiss goodbye all that lucrative federal government work. JPL goes away, Boeing goes away, all the defense contractors go away, the US military bases go away. How big is that economy really when you take away the federal government? The California economy is the result of vast amounts of federal investment including silicon valley. I'm sorry but the reality is that the federal government is so deeply entwined with the Californian economy that to speak of them being the 6th biggest economy in isolation only shows a deeply flawed analysis as does the simplistic measure of "taxes out" vs "welfare back" definition of being a "net contributor".
It is pretty arrogant to think nothing changes should California prove itself to be nothing more than just another unstable country. Any place that wants to change its entire government simply because the elites didn't get their way is the very definition of unstable.
In the end, a Cal-exit would serve to weaken the U.S. but it would likely be fatal to California. Finally ask yourself this, if California can support itself agriculturally why does more than 80% of the produce in my local LA area market come from Mexico?