>Population of Seoul metro is 26 million people. It's suppose to drop by 2/3rds by 2060 (or worse)

This doesn't make any sense at all. The median age of Seoul is 42, slightly younger than the national average. 2060 is in 35 years. If Seoul had zero births, zero immigration, most people in Seoul today will still be alive by 2060.

I feel like every time someone comes up with one of these predictions the numbers get more nonsensical.

Yes, "most people" will be. But they lose 0.007% a year and only replace 0.004% a year. This year 189k people will die, 114k people will be born.