I am actually more worried about Kim Jong-un dying

Although Kim Jong-un is on the same side as China, it is known that he is equally wary of them.
The real problem would be if, after Kim Jong-un's death, a pro-China military faction takes power instead of the Paektu bloodline succession and immediately moves to discuss unification.
If they kick out the US forces stationed in Korea claiming that they are no longer needed since unification has occurred, I don't know if we would even be able to defend ourselves when China rushes in.
This is personally why I feel that maintaining the status quo is safer than unification.