Great point. 4th largest military just north of the border, with an ass load of heavy artillery aimed at a city of 25 million people... and this is about The White Man's Burden. South Korea and Japan are allies. Maybe you forgot about that.
Not that I agree with Jason's point of dependency, but....
SK only spends 2.7% of its GDP on its military. Clearly they are not that worried about the long term situation. For comparison's sake, Israel spends 5.8%. IMPOV, SK has a sweet arrangement. They let the US stick one of its feet on their peninsula, and be the big bully providing them cover, while Samsung, hyundai, Kia, et.al. export tons of shit our way. And the US military-complex gets to keep two of the sow's teats completely to themselves.
Of course now NK appears to be on the brink of becoming another nuclear weapons power, which naturally scares a lot of people shitless. So we don't get to ponder what would have happened if the US had let SK start handling its own affairs some 20-30 years ago...
A crazy idea that our military-complex wouldn't tolerate, but the PRC might agree to: We have secret discussions with the PRC, and agree to a joint attach on the fat kid, we hit the DMZ as hard as we can as well as ICBM sites, and the PRC topple the fat kid and sets up a new friendlier kid, but neutered of nuclear technology. Once its over, all US military permanently leaves the peninsula, with SK free as always to buy US toys.