You are exactly right. It is very doubtful that the left has thought this out very far.
Cut off food and fuel deliveries and electricity, which is not difficult, and the cities turn into cauldrons of anarchy in hours. Most city dwellers are painfully lacking in survival skills in such a situation. These are people who call 911 when Popeye's runs out of chicken. They wouldn't be able to start a cook fire let alone prepare food on it.
Within a week any governmental authority that remained would not be able to cope with the dead bodies.
Why would we stop trading with California when we trade with Communist China?