Could've, would've, should've. The US has had a long and sordid history of downright fucked-up foreign policy decisions.

But, bitching about it doesn't really change the current situation. NK has a dictatorship with a enough military power that it still requires a decent army to overrun. Kim isn't going to listen to any diplomacy, except when he can trick some diplomat to give him more power or image-building. At best, China might be able to cut off its funding and topple Kim's little empire, but China has no interest in that.