First, they seem to miss the fact that such an action would leave the United States with no west Coast Ports. Oh, they would allow commerce through, as long as the items met their strict environmental standards. This being said I am reminded of how often the port of New Orleans was closed by Spanish officials to American Commerce until the Louisiana purchase. One disagreement between the new government and Washington and you could expect port access to be rescinded. And the Billions of dollars in losses to East Coast Liberal Millionaires and Billionaires would be heard all the way to Europe.
Again, regarding Port access, the US Navy would literally be incapable of operating in the Pacific without Puget Sound or Coronado. And I suspect the leaving of the union would also prompt a pacifist push by the Ultra Libs to remove the few remaining military bases on the West Coast. This would make extremely difficult our supporting of our Pacific allies- Japan and Australia- as well as the defense of California, Oregon and Washington. I would expect within months following a required US military pull-out military forces from Russia, China and even Mexico would enter the State. What would be there to stop them, the San Jose Police Department? Even the Cal Guard exists only because of the support of the United States government. All the weapons would have to be returned to the United States, the actual owner. And California's own Assault Weapon law has provisions to prohibit the State from raising secondary troops: out-side of the fact they would have no arms to give them because of the quarter century prohibition.
I particularly like the remarks of how the United States would collapse if California left since they had all the tech people. Most engineers I know are getting ready to leave California because the companies were leaving to avoid the high taxes and general costs of the State. The West Coast may be the center for Software manufacture (if you can call it that), but the making of physical items is mostly done out of State. Even Big Bad Boeing has threatened to move all its engineering off shore and has farmed production of key components to Japan and Europe. I doubt a secession would make them want to bring the work back, or slow down the present rate of withdraw.
Finally, The movements backers (mostly Software types) seem to forget that there are a number of conservatives in California and more then a few Democrats who would not want to leave the United States and US Citizenship. 3 million Californians voted for Trump, one third of the votes cast. Even if the total number of conservatives was 6 million, that would be 20% of the population. I doubt California would find such a loss of people a blessing. They would bring with them all their skills and knowledge and start new companies to fill the gap caused by the California exit. Copyrights would only be valid if they were international.
In short, outside of the general legality of leaving the Union, West Coasters would more then likely find themselves without jobs, without property, and living under either foreign occupation, or United States protection.