Most states just live off the teat of the national government, and just have "their" National Guard units (funded by the US government...all weapons and equipment provided by the US national government...paychecks of soldiers from the US government). Only when the governor activates them does the State pay anything. The US allows the states to activate these units for emergencies, but reserves the right to nationalize them under certain conditions, and an invasion is not required...war is not required. Ever since the end of the Cold War, national guard units have become more and more important to the national defense of the United States, and they are activated by the national government fairly frequently. Just for example, the Nevada National Guard just returned from deployment overseas, even though there was no war they fought in. One would wonder if that was illegal in the San Francisco court's eyes.
Texas is an interesting case, in that they have both (1) a Texas National Guard (federal troops, funded by the US national government, like all other national guard units), and (2) a Texas State Guard (funded entirely by the State of Texas). If California wants its own Guard, not subject to nationalization by POTUS, then perhaps it should fund an organization like that. I will be shocked if SCOTUS limits POTUS power to nationalize Guard units in any way. If it does, then I suspect Guard units will just be absorbed into the Reserves, and if any State wants the capability of activating a Guard unit, it will have to form their own State Guard units like Texas does.