“Yet, why does SK need 40,000 Americans over there as a trip-wire?”
That trip wire is itself a huge defensive action because if tripped by North Korea it means South Korea has much more than just its own means at its disposal. There is every possibility that that trip wire has caused North Korea to NOT initiate another war.
In 1998, I did a two-week TDY with my National Guard unit in Seoul. I ran into an old American Sergeant and asked him just this question.
His reply was that while the South Korean Army is well-trained and can fight savagely, their senior officer Corps is political and not merit-based, and are indecisive when having to make quick decisions.