Stagnation in terms of total economic output, maybe. But again, why does that matter when the quality of life and income of a typical Korean will improve drastically from a lower population?
Developing countries can keep reproducing at such a high rate and boast of their total economic output, but what's the point when most of the country is poor as dirt?
The only people that benefit from high populations are large asset owners that get richer through higher asset valuations and lower incomes. Do you own factories in Vietnam? Do you own lots of real estate there? You probably don't, so why would you want a higher population figure?