Our economy and intellectual progress is massively predicated on specialisation of labor. When I consider any single aspect of our life, even something as "simple" as how elevators work, it's astounding the amount of complexity that goes into it. Complexity that we can handle because we divvy it up amongst many many people who are completely specialized in understanding and fulfilling that task. Gradual increases in…
As for population going to zero, I suspect it'll be a while -- and that we could increase birth rates with financial incentives, 2-3 years parental leave wouldn't be very dystopian :)