Standard of living is supported by those who can produce versus those who cannot. As population declines the demographics skew to mostly be older non-working people. There is a certain point where the percentage of people working versus not working is too small, then the economy can no longer produce enough for everyone’s current standard of living. It can range from relatively minor case of not being able to get all the variety of food, or it can be major where people starve because not enough food can be produced. Or medicine, or care, or electricity, or oil, or plastic, or TV shows, etc.
Given enough time a new equilibrium and standard of living comparable to the old one will likely result, but getting to that new standard of living can mean people died.