You may be right. But still, three minor points;
1) N.K. has nuclear weapons
2) It is (collectively) 'barking mad'
3) There's no oil to get.
You forget the single most important point- Uncle Qin sits over the Yalu river waiting to repeat the last attempt that the Free World made to get rid of North Korea.
It's not two of the three points that you made which keep us from attacking, or have ever kept us from attacking... it's the plain and simple fact that we have not no nor have we ever had the desire to fight the millions of Chinese that will once again stream across the border as reinforcements.