What is the Chinese perspective?
I consider NK to be a serious threat to the US and her allies but I think the Chinese do not consider NK to be a serious threat to the Chinese.
Imagine if Cuba decided to start allowing Chinese anti-missile defense systems at the US' doorstep because we weren't willing to deal with our ally Mexico regarding a conflict that didn't threat us.
The missile defense systems we deployed in SK could be used to track Chinese missiles. My point was imagine the outcry from the US if we perceived that the Chinese were tracking our missiles under the guise of tracking Mexico's.
It only makes sense for threatened parties to develop defense if not retaliatory measures. Imagine if the USSR never developed nuclear weapons while the USA threatened them with nuclear attack?