We should have had our troops out of South Korea decades ago. South Korea is no longer an impoverished emerging country. It is a full-fledged industrial power. Our troop presence there is very unpopular among the Koreans.
It's time for them to take out their own garbage.
Unfortunately, we can't do anything now, not after these threats. We can't pull our troops without appearing to be folding to blackmail. We can't attack North Korea because the South would be totally opposed. They prefer we just sit here and see if he's bluffing. Win-win for them.