The issue isn’t the population numbers per se, it’s the population distribution - I.e. that it’s going to be a population of mostly older people who require a lot more help from the state in terms of pension, healthcare costs, disability benefits etc. mixed with much less workers to generate income for the state.
South Korea is already past the point where it can’t reverse the fact it will be a country of majority over 65s in the future. Many of whom end up it cramped old people’s homes which are already well over capacity.
Then there’s the issues of democracy. How can you change these things when the majority of voters are all over 65 and crammed in retirement home. They have a right to vote for their own interests in terms of pension, state funded healthcare etc. But you can’t realistically provide these things without large scale immigration from Africa and the Middle East, which is where the majority of young people will come from (which itself is becoming politically impossible because immigration has become so politically weaponsised) or making people work 60+ hour weeks.
“The natural ebb and flow” also usually means brutal world wars, pandemics or inhumane governments trying to change a populations distribution through force I.e. forced deportation or worse of certain demographics. But lots of suffering regardless.