The population doesn't just go from 100m to 50m instantly. It gradually changes over time, 100 years from now the smaller population will work itself out, but none of us will be alive for that. We still have 100 years of discomfort to get through.

Fewer children mean all the industries and gov't services who are employed now to service children will need to downsize, these are lost jobs now before the fewer children grow to adults where they would take over those fewer jobs. All of this will have a effect across the economy.

Pediatricians, Teachers, Toys & Games companies, Children Furniture, School Supplies, Electronics, etc.... All of these are sized with the expectation of the same consumer demand, but when there are less kids to buy and service each of these will be forced to downsize. Again in the long run it works out, but in the short run say next 50 years for people in these markets will see downsizing over time. Can the rest of the economies pick that up?

Pediatricians - not enough of them now. Teachers - also not enough. Toys & Games companies - can switch to adult games, no problem. Children Furniture - can and do make adult furniture too. School Supplies? Electronics? Can also switch to adult products. Fewer people means fewer companies as well - not a problem.

Lots of new jobs will be needed in health and elderly care - you just ignored those.

> All of these are sized with the expectation of the same consumer demand, but when there are less kids to buy and service each of these will be forced to downsize.

Downsizing happens all the time. It's considered normal by most economists.

Yes, there are domains that are more affected than others. Reduction in population is slow enough to simply let the workers retire without hiring new ones.

You're acting like the birth rates are 0.2 instead of somewhere above 1 (too lazy to check). I remind you that Japan had lower birthrates than that for a very long time and nothing bad happend. In fact, their workplace conditions are improving. Salarymen are finally starting to work decent hours.

Why don't you admit that you're worried about your own quality of life at 70y old and you couldn't care less about future generations and their polution, resources, global warming, famine and refugee problems?