Let's not forget the real purpose of the Warsaw Ghetto.
Hong Kong is already a political problem for the Chinese, and it is at least economically successful. Why would they want to trade one political problem for another? Why would they want to sponsor North Korea economically in ways that put their own people at a disadvantage?
China has been perfectly content to let North Korea continue the way it has. The only thing that has changed is the level of bad behavior displayed by the North's regime.
Btw, the rich are getting richer on the backs of the poor. 'Twas ever thus. China has plenty of its own.