Korea, South Korea, is at the top of its game. That's not to say that a fall is imminent, nor even on the horizon, only that their current degree of engineering prowess and economic prosperity is unprecedented and, to use an over-worn expression, world class.
The other Korea, North Korea, by contrast, is decades behind the world community in almost every respect, a situation compounded by the malnourishment of its people.
This set of circumstances is very like that which existed in Germany (then West & East Germany, respectively), just prior to the dismantling of the Berlin Wall. For West Germany, reunification was initially a disruptive burden, owing to the poor economic condition of the east, but it also meant a suddenly expanded labor force sharing a common language.
In Korea reunification seems a remote possibility, at present, but a shift in the political wind in the north could change that rather quickly. And don't expect much in the way of resistance from China, since North Korea's persistent cult of the leader makes communism look worse than it otherwise would, and reunification would provide them with a more prosperous trading partner in place of the burden North Korea currently represents to them.
But the really interesting dynamic would be how the economy of Korea as a whole would benefit from reunification, through the combination of North Korean labor, which could be made far more potent by means of a little food aid, and South Korean industrial ability, which, automated as it is, can still make good use of less expensive labor.
Even more than with Germany, language would bind and lubricate Korean reunification, since the Korean language is not widely spoken outside of the peninsula and also not strongly subdivided into dialects. Even without exclusionary laws, the common language would help insure that South Korean business had special access to the North Korean labor pool.
One major question remains; can some degree of economic reunification proceed without a regime change in the north? If China were to signal that North Korea should look to its southern sister for assistance, that seeming impossibility would suddenly become a matter of how much how soon, and for China that would be a matter of shrugging off a burden.