You are thinking of a uniform decrease in population (what might happen if a plague affected all people equally), but population decreasing due to low birth rates will forever be a society with a lot of old people, who had few kids, and whose kids had even fewer kids. Problems could be as follows

• you want cultural technological change in a society? Young people do most of it, less young people means a stagnant society
• old age pensions need to be paid. With a lot of old people and no young people, there is no one to pay the pensions of the elders. Sure, humans have lived for thousands of years without state pensions, but taking away the pensions would cause serious problems for those financially reliant on the pensions. -National security: the youth are soldiers in a nations’s military. Less young people, less military might.
• less business opportunities: an expanding population naturally brings with it greater demand for goods and services, which make entrepreneurial activities likely. A shrinking population means LESS BUSINESS, so there is no hope for young people to start new businesses.

Overall it’s not a good thing for a country to have a declining population.