It is not just the dependency ratio. When your population declines enough, you end up having trouble maintaining all sorts of infrastructure. That is especially bad in rural regions. If your population density falls, that means fewer people have to pay to maintain basically the same length of roads, electricity grid, water system and so forth. Fewer customers leads to shops and restaurants closing. With fewer young people, schools will close making even more young people leave, as it makes raising children that much more difficult. So larger villages and small towns tend to do somewhat fine, but villages end up with pretty much no young people and just die. Even worse with an overall population decline the biggest problems of cities, namely the high cost of housing becomes less of a problem.

It really is not just the dependency ratio, which is a problem. In fact that one is often stable, as old people die.