Why?

Why does China continue to support North Korea when it is widely known that the country is one of the most corrupt and repressive nations on earth?  The answer that China wishes to preserve communism hardly suffices because even Chinese communists learned that repression doesn’t work well and ultimately, ruins a country.  North Korea is a nightmare.  It doesn’t listen to its people whom it starves and imprisons on whims.  It deifies its rulers and forces the public to celebrate them.  It uses the propaganda techniques that George Orwell satirized 70 years ago.  Surely the Chinese understand this.  Yet they persist in upholding the country’s leadership.  It wouldn’t be long for North Korea to survive if China closed its border and stopped all trading.  China could be looked upon as a hero to North Koreans if it took over the country and opened its economy as it has done at home.  Yet, it doesn’t.  There is political inertia at work that makes no sense and North Koreans are left to suffer.