[smsm42]

They stay in existence mostly because nobody wants to take the responsibility for 25 million people that are stuck 70 years behind. The West is in no shape to do any nation-building, neither financially nor ideologically, South Korea has no resources to stomach such huge influx of the population with such huge economic disparity (it's like US would get 150 million immigrants from extremely poor country all at once), and China is completely fine with the position of NK being a permanent toxic dumpster fire and China being seen as the only power being able to manage it and thus being irreplaceable and everyone's savior.

They are as "successful" as cancer is "successful" - it's hard to kill the cancer without killing the patient, but that's not a redeeming quality for cancer. In the same way, it's hard to kill Kim's regime without subjecting 25 million people to extreme dangers, exacerbated by the fact that many of them are already on the brink of starvation, and a lot of them are thoroughly brainwashed. It's not a sign of "success" or regime's rationality, it is the sign that it is hard for us to find a solution for this without hurting a lot of people in process.