> Population of Seoul metro is 26 million people. It's suppose to drop by 2/3rds by 2060 (or worse). That's 1/3 less taxes, a 1/3 less transportation riders, 1/3rd less customers, etc....

If there are only 2/3rds as many people, why would the same infrastructure be required?

The problem is if you have overbuilt infrastructure, it doesn't just magically go away. You either need to pay to get rid of it, pay to maintain it, or deal with the consequences of letting it decay. Further, with an aging population, the decrease in their ability to pay for this infrastructure doesn't neatly balance out with a declining need for infrastructure. The retiree in poor health is more reliant on public transit, more reliant on healthcare, etc, while they both have less money to directly pay for those services, and contribute less to the economy as a consumer of manufactured goods or as an investor.

There is nothing special about any population number, it's how quickly and in which direction your population is changing that matters.

There's an infrastructure inflection point. Note how cities have much better infrastructure than country towns.