> other than supporting aging population?

Isn't that enough? Imagine a world where a large percentage of the population are in nursing homes. A humane goal for a nursing home is 10:1 24/7. So that means 1 nurse for every 2.5 residents.

Besides that, it's all about the speed of change. Current Korean levels of population halving every generation is going to cause tremendous upheaval.

Shrinking and growing populations aren't necessarily problematic. What's problematic are populations that shrink or grow too quickly. Infrastructure adapted for N people works well for a number close to N, but not so well for 2N or 0.5N.

There aren't too many people besides Elon Musk that are significantly worried that the US's replacement rate is 1.8 compared to the 2.1 constant population level. But numbers much below that do alarm many.