M3G4MIND t1_jdv8oyz wrote

Because you don't have an even distribution between old and young people. In your example the population decrease from 100 to 80 would mean a drop in ice cream workers from 25 to maybe 15, meaning they wouldn't be able to make enough ice cream cones for the 80 people. It's the general issue that most of the western world is facing. Less young people to carry the work load and more old people requiring resources.