Predicting population growth out 100+ years is unlikely to be accurate. South Korea is likely to have a larger population out as far as we can reasonably predict.
Long term, the people born each year are children of people who decided to have kids which is counter to the overall trend in the country. There’s huge selective pressure around deciding to have kids in modern societies.
If there's far fewer parents because this cohort of potential parents is much smaller, then you have necessarily fewer children going forward. Even if you double the number of people that want kids, it becomes very hard to reverse the demographic decline, let alone grow your society. Even if you could, there's still a giant demographic hole in your population that will cause big issues over time.
Outside of immigration (which is a big discussion on its own) PLUS a healthy birth rate among these immigrants, I'm unsure where your optimism comes from for them reversing their demographics and growing.
Immigration is a very dificult topic in both Japan and South Korea societies. The fact that EU is becoming increasingly anti-immigration won't help to advocate for this.
Total population has inertia, but anything more than +/- 2% out 10 years is pushing it and that uncertainty just grows with time.