China does not want U.S. troops in N. Korea period, therefore, it will pre-empt any justification for U.S. presence by formally (via UNSC Mandate, or unilaterally) occupying key N. Korean nuclear infrastructure installation with Chinese special forces.
It would make any attempt by U.S. to insert herself in the N. Korean drama to be unnecessarily destabilizing, as most of the rogue nuclear material (WMD pretext so often used by US) would be under the very safe hands of PLA special forces.
There would be no joint-enterprise to begin with - China is trying to evict U.S. influence from Korean peninsula, it's not interested in jointly eradicating the only paper treaty alliance it has on earth.
It would make any attempt by U.S. to insert herself in the N. Korean drama to be unnecessarily destabilizing, as most of the rogue nuclear material (WMD pretext so often used by US) would be under the very safe hands of PLA special forces.
There would be no joint-enterprise to begin with - China is trying to evict U.S. influence from Korean peninsula, it's not interested in jointly eradicating the only paper treaty alliance it has on earth.