Bingo!
The US threat (in regards a conventional war) is our ability to achieve total air dominance. While the Chinese air force is pretty big, and has decent aircraft, we are bigger, and better.
I remember seeing news articles during the NATO bombing campaign in Serbia. People were saying that our planes were going to be getting knocked out of the sky by the top tier former Soviet Air Defenses.
This did not happen.
Our ability to make it impossible for the Peoples Army to move men, food, and ammo to the front is what scares China (and should, as it’s exactly what we would do).
As things sit, China can use North Korea as a buffer and a bullet sponge while they get their act together. Unified US friendly Korea puts our gear right up on the Yalu river, and they don’t want that.