Foreign policy should reflect international realities, which change over time. The Republic of Korea was vulnerable to renewed North Korean aggression at the conclusion of the Korean War in 1953. Today, Seoul could do whatever was necessary to deter and defeat the Democratic People's Republic of Korea. There's no need for America to defend that which could be defended otherwise.
Bernie Sanders's problem with exports, "because they benefit large corporations," is more than somewhat astounding. Who does he thinks works in all the Boeing plants? Doesn't he realize that a large percent of Boeing shares are owned by various American retirement funds and individual American 401k plans?