Japan's population started declining at 0.5% a year in 2021. It won't be until 2039 that South Korea is expected to decline at that rate (though if their birth rate stays at the current level it will likely be before then).

Though taking a closer look at South Korea's population pyramid maybe it'll take another 20-30 years, since their population won't have declined as significantly as Japan's until then.

This seems like the most likely answer. I think people pay more attention to the current population pyramid than the birth rate. I expect there's a good chance South Korea will take Japan's spot in popular culture within the next 10 years.

Thanks for letting me know. Didn't realize that.