CA has the 7th biggest GDP in the world. I think they will be fine.
If this far-fetched idea ever came to be, who is to say the rest of the nation would let the entire state go? Sure, economic hubs like LA, SF, and Silicon Valley would go as they are the most left wing areas of the state. And that would provide the breakaway state a strong base for its economy.
But much or most of inland California is red. Why, in this highly unlikely scenario of secession actually happening, would the rest of the union allow the coastal secessionists to have the whole state? If it’s about divergent values, then those red counties (with significant agricultural and mineral wealth) may wish to remain in the union, and the union might insist that they be allowed to do so.
Insanity knows no bounds ...