I've spent the last past decade pointing that out to people. People act like a state being separate from the US and independent is soooo absurd, logistically.
But the US can still assist on a natural disaster just like it does in all of the Americas. Common defense can occur, as you point out. Visa and waivers could be easy.
I always like to imagine what level of state tax would I tolerate if I was only paying income taxes to just California. Since California doesn't get much/anything back from the Federal Government from its remittances, I bet California wouldn't even need to raise its states taxes.