Juche doesn't work. The theory of comparative advantage is not wrong.
If economic actors can choose what comparative advantages they possess (e.g. by moving the economic factors or through training or investment), the theory becomes meaningless. It turns into a predestination fallacy since it myopically focuses on geographical factors that happen to not matter that much in the modern world.
What economic advice would the theory of comparative advantage give to China in the 1960s?
If it is "just do the free trade", it would have been wrong. China started to implement its market reforms with dual pricing to protect its own citizens from the foreign markets.
Now that is an actual comparative advantage that is actually hard to replicate due to huge entry costs.
(To lower the costs of labor further you need to actually give out life basics for free! China gets away with this by having a huge population competing thus far, but it won't hold.)
It's unambiguously true that free markets has made South Korea significantly, mind-bendingly wealthier than the North. And on the subject of fertility, birth rates are so low (1.8) that you have Kim Jong Un crying on national TV, begging women to have more kids.
Personally, I have this pet theory (irrational, it may be) that as the boomers die off across the West, newer generations will pick up having more kids and TFR will level off. Like new growth springing up in a forest after a wildfire burns up all the old, dry wood.
Something similar to the post-war baby boom.