What if China decides it is in their interests to "take care of " Kim, and replacing him with someone more to their liking ? Kim rules by fear and I imagine he has made lots of enemies (the ones he hasn't fed to dogs). I would bet that they (China) have people on the inside and will act when China deems it necessary to their interests. How many of those closest to him want him gone ? They just need someone else to do the honors. 
China must be monitoring the situation, maybe even enjoying it for the moment. 
China is the power and China may be the only one who can stop a eminent disaster.
It will cost us, the Japanese and other allies something but pretty sure it will be worth stopping a H-bomb or A-bomb (whichever it is ).