There are no really good options.  Sending planes into North Korean airspace would certainly prompt retaliation, probably in the form of shelling ROK bases, maybe even Yongsan.  And then what?  Back down?  Escalate?  How do you think the South Koreans feel about America fighting to the last Korean in order to protect Los Angeles?  Demilitarizing the peninsula might be necessary at some point in order to secure Chinese cooperation, because China won't accept American troops on its border.