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.
If America ends up at war, it almost certainly will be on behalf of one ally or another. Washington collects allies like most people collect Facebook "friends." The vast majority of U.S. allies are security liabilities, tripwires for conflict and war. Alliances should be based on interest, not charity.