Again, NK is not East Germany. It would be far more like incorporating a stoneage society in many respects. If NK implodes, look for SK to maintain the DMZ and set wickets for economic reunification. SK will want to bring the SK economy to the North, vice having NK's flood into SK.
The U.S. will have no contructive role in helping NKoreans directly until they've had time to socially adjust a bit. Even defectors are frightened of Americans from what they've been taught since birth. There is also the issue of an entire generation that is physically and mentally stunted from malnutrition.
I do not disagree, except that it is a matter of when, not if.
We would do well to follow the same model on our southern border-- the Mexican Americans who have made the successful transition to Americans would be invaluable in bringing the American model to Mexico.
Open borders would simply drag South Korea down to a level only slightly higher than where North Korea is now. Higher only because the South Koreans have the know-how to pull themselves out.