Ok if that’s the reason then why is/was China against North Korean developing nukes then, to the point of joining the US to impose sanctions on North Korea and isolate them? So you think China has been unfair to N.Korea? lol. China knows it’s also a long term threat for them especially an unpredictable country at its border . Plus nobody knows if tomorrow day Kim regime will turn against China herself . Interests changes in geo politics all the time . China knows this.
Plus China has done her best to be rational with that paranoid brutal dynastic regime of Pyongyang who kills her citizens who dare emigrate from the country. Even a fool can see that the regime in Pyongyang is paranoid to the point of being crazy. Who kills her own citizens who try to move to another country ? Is that not madness ? China has tried many times to bring Kim dynasty out of their self imposed paranoia and isolation but the regime has refused time and time again . China has tried to show them a better way and open them up the way China did in the 80s to this day but the Kim dynastic regime wouldn’t have it. In fact , fat Kim went to the point of eliminating all pro Chinese elements in his regime including his own uncle. Shows you the current regime has Absolutely ZERO trust in China . In fact North Korea is even closer to Russia today than to China.
Anyway, the way the regime in North Korea is going, we all know they can’t keep up like this forever, the whole world has moved on but only North Korea has remained in her isolation(even Vietnam communist party opene up and reformed ), the Kim Dynasty is bound to collapse one day. Their system is unsustainable. The day it does, this can be an issue for China.