the difference is that most western countries have land borders with other western countries which have some degrees of freedom and prosperity, while south korea's only land border is with the most totalitarian country in the world. canada having a low birth rate does not put us at risk of being conquered by the us. france having a low birthrate does not put you at risk of being conquered by belgium or spain. south korea is in fact prime conquest material for north korea if the situation appears and china would love to help north korea do it.
Canada