Why is rural depopulation worrisome? Young people, as one would expect, want to be located near other young people and jobs.

Increasing the high-density urban population leads to even lower fertility.

"We find a robust association between density and fertility over time, both within- and between-countries. That is, increases in population density are associated with declines in fertility rates, controlling for a variety of socioeconomic, socioecological, geographic, population-based, and female empowerment variables."

https://pubmed.ncbi.nlm.nih.gov/34914431/

These young people will be middle aged people located near old people, with a huge average age - and the flee to the cities will increase that.

Because that's where the food comes from.