I know this is meant to be tounge-in-cheek and not serious, but still fun to mull over.

IMO, main issue is the assumption that GDP/capita is independent of population size.

I suspect most economies would hit a saturating point where most people are underemployed and there's no productive work left for new entrants.

While South Korea shrinking 1/4x each generation is bad, growing 4x would probably also be bad.

Seems like the best case would be hitting some optimal population size + age distribution then maintaining that each generation -- not shrinking but also not growing.