The ability to leave the union would imply the ability to return or to further subdivide what’s left and then return.
I would expect San Diego for example to do so, after considering the ramifications of losing the US Military. Also the rural agricultural areas.
Don’t forget, a seceded California gets to take its share (~35-40%) of the debt plus whatever unfunded obligations its population has engendered.
The military base issue is a red herring. The US military has military bases all over the world. Military bases in England, Germany, Italy, and Japan have been US for nearly a century.
under CALexit, Federal military bases are likely to remain.