> Even at a time where it had become clear that the apartheid system could not continue
If North Korea can continue apartheid sure as hell could have. If you're willing to kill all the people to keep your power you won't need to. Compare, Stalin, Lenin, Gorbachev, Deng, Fidel.
That's a comparison I hadn't thought of. But the difference is that North Korea shares a border with its patron state, China, which is also a global super power.
While South Africa had the support of the United States, it was thousands of miles away from the US, and surrounded by Black African-run states that were hostile to it, particularly Mozambique and Angola. South Africa did not have the resources to run a North Korean-style police state.
North Korea has never been as rich per capita as South Africa was at the beginning of the apartheid regime and South Africa had nuclear weapons a long time before North Korea did. If they had wanted to keep the US out they could have, never mind African militaries. The nearest military that isn't obviously inferior was in Israel.
If North Korea can continue apartheid sure as hell could have. If you're willing to kill all the people to keep your power you won't need to. Compare, Stalin, Lenin, Gorbachev, Deng, Fidel.