> how do you sell 100 million smartphones when the population is only 50 million
You don't. You only sell 50 million.
> this leads to layoffs
Why? 50 million people instead of 100 million also means half the employees in the factory, making just 50 million phones instead of 100.
> government offices cut bodies (layoffs) and reduce services
Yeah, but no layoffs (same reason as the phone factory). Fewer people need fewer services. Potholes are indeed a problem. Some roads leading to abandoned places will need to be abandoned as well.
> most stocks will tank
By your argument, those people that can't buy a second phone, also can't buy any stocks anyway. I see no problem here.
Potholes are indeed a problem
actually, less people would mean less traffic, and less wear on the roads, therefore also less potholes.
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?