there is little appetite for warlike action that would disrupt the South Korean economy or destabilize the North enough to require the South to take it over.
Robert Kaplan wrote about this in the
Atlantic, and pointed out that the projected costs of reconstituting the civil society and infrastructure of the DPRK after the fall was so horrendous that the ROK and United States are very hesitant to even move in that direction. The Japanese would be shut out of any meaningful role in the reunification for historical reasons, and China and Russia would both work to find ways to benefit at the expense of a tied down American/ROK nation building effort.
Greater Korea would probably discover the port and transport infrastructure of the Tumen River region have been absorbed by China or (long shot) Russia during the turmoil of reunification, and no doubt many other nasty surprises would be waiting for would be nation builders. On the other hand, "Downfall" is pretty much inevitable in brittle, authoritarian regimes, so someone, somewhere should be making plans.