As others have noted here--and I noted myself from the get to--was that the new Snowflake Republic would have a built-in insurgency yearning to return to the U.S.
Secondly, someone was making a point about the Silicon Valley types thinking that they would be able to continue to do business as usual. Wrong--a blockade (pardon, a "quarantine") is an option for the federal government. This quarantine could also be applied to cyberspace. What happens to Google when NSA Hackers go after them with a vengeance?
Thirdly, U.S. forces could block the flow of the Colorado River into SoCal. Couple that with pro-U.S. militias in the high country holding the water sources, and things will get desperate real fast.
In point of fact, I have been pondering for over a year a board wargame about a 21st Century American Civil War. I guess that I need to think about a California scenario now.
But;
At least Texas has (mostly) it's own power grid. California imports a lot of electricity, and with the impending closure of the Diablo Canyon nuclear power plant will soon be importing yet more.