The so called "Hermit Kingdom" of North Korea for a long time was an object of ridicule. Country that bans internet, computers, mobile phones and other modern equipment sounds more like a joke then a viable state that can survive and function in modern world.
Many experts predicted that this impoverished starving state would soon collapse, and Korea would re-unite under Seoul government. DPRK however managed to defy these predictions and continued to exist until this day. As of now demise of Pyongyang regime looks ever more distant, despite its poverty and inhumanity it continues to cling to its existence. That raises the question: why?
The answer to that is rather straightforward, DPRK has a powerful protector in form of China that keeps propping them up and prevents their demise. Every so often another powerful protector in form of Russia helps out as well.
However, why would China or Russia bother to prop this unstable and inhuman regime? Isn't North Korea a danger to everyone around them with their nuclear experiments? Aren't they abhorrent tyrannical regime that is afront to everything good and just? Wouldn't it benefit China to instead have a direct border with South Korea for easier trade?
These arguments may be true to the west, but China does have a peculiar reason for supporting continued existence of DPRK. The reason is propaganda and information control.
Chinese are very envious people and care deeply how much they are better off compared to their immediate neighbors. Being "better than Joneses" next door is crucial to their continued approval or disapproval of their government.
That is why China runs its infamous Great Firewall of China. Its purpose is not so much as to prevent people from mocking leaders so much to prevent them from learning about places like South Korea, Taiwan or Japan. China wants to be seen as indisputable leader of the Asia and uses its great firewall to filter out all information that contradict this image. Information such as quality of life and prosperity in three Asian democratic countries.
Because China borders only impoverished DPRK, Vietnam, Laos and Myanmar who all look to China for guidance allow CCP of China claim that they are better off than all their neighbors. South Korea and Japan are physically too far away to allow ordinary Chinese to easily compare these countries to their own.
In contrast having a direct border with RoK, that not only has friendly relationship with the US but also much wealthier than PRC would make Communist Party of China look incompetent in its people's eyes. Chinese public would start questioning their government and one-party system. They would ask things like why we can't have democracy like those Koreans and prosper as much as they do? After all what else South Korea has that China does not?
That would put current rulers at risk of losing power and they do not want that. Because of that they keep spending increasing amounts of Chinese money to keep DPRK somewhat alive and prevent complete collapse of Pyongyang regime. China needs some impoverished loser next door to feel good about itself.
However, while China needs DPRK on its border, it does not mean it needs DPRK to be in its current form and size. A complete disappearance of North Korea and direct border with Seoul government would be a step too far for Beijing, but a rump North Korea that continues to hold some land on Chinese border, preventing direct border between RoK and PRC might just be enough for Beijing. This rump North Korea would then continue to function as part of Great Firewall of China just as before. This can be combined with some areas under direct Chinese occupation as well as no man's lands.
While this solution is not as good as complete unification of Korea under Seoul government, it is at the same time better than continuation of the current system. North Korea's most populous areas lie in the country south, close to its border with RoK. Annexing just these would allow making lives of millions of Koreans much better and safer.
As to where the border should then go, then perhaps mountains in the very north, close to Yalu River would be good enough to China. If not these, then the area between Sinanju and Hanju, where the Korean peninsula is thinnest, might work. The borders can be discussed with Chinese directly.
This partial Korean re-unification is more realistic than a complete one and can make lives of so many people in Korea much better. That would make South Korea larger and more productive on the world stage. I think its a goal that diplomats should attempt to reach.