I was in Czechoslovakia when it split into two countries. There were some bumps indeed. But things worked out reasonably well.
When USSR split, Russia "leased" the naval base in Sevastopol from Ukraine. Agreements were somehow reached. San Diego would not disappear overnight.
If California becomes its own nation, it would have access to its own central bank, which the state (now national) government some tools to handle its economy. . Plus it could default on its current loans, or use the threat as a bargaining chip.
A while back, there was a proposal to divide California into three states: NW, SW, and Interior. Each region kind of has its own unique culture--and may just be more governanble than one big splawling state. I thought the 3-state was an idea worthy of consideration.