North Korea can't trade its goods outside and is dependent on foreign aid. Its people are subject to famine, like they're in Africa or something. After WWII, there weren't instances of mass starvation in East Germany (mass shortages, definitely).

All I'm trying to get at here, is that rebuilding North Korea would be a major undertaking beyond East Germany and look at how expensive that was. Change can't come as suddenly, either, as the North Koreans are built into a personality cult. For them to discover that it was all a lie overnight would be devastating.

Like I said, it's going to be some North Korean general that finally kicks out Dear Leader and puts the country on a path similar to China with reunification to happen way down the line. They have to get their economy and international standing back on track first.