The median age in Japan keeps rising. In 20 years the average age will be above 65. Someone has to care for the people aged 80 and above, and if your country has 40,000,000 retirees compared to 30,000,000 people of workong age, that's a an impossible circle to square. Japan and South Korea are simply not going to be able to properly care for their aging populations. Having 2 people turn 65 compared to every one that turns 18 is a massive problem that can only be solved through immigration.

Capitalism has definitely made the problem worse, but the developed world is about to hit a demographic cliff that is going to tank the quality of life for billions over the next few decades.