1 billion people is more than enough. The years 1865-1905 were some of the most amazing years in human history, and there were only ~ 1 billion people on the earth.
and all these worries about having too many old people are way overblown. Any problem whatsoever which involves 70+ year old people is a short-term problem.
Pick any metric. Human rights? We got rid of slavery. Technology? we mechanized farming, first with steam engines and then with the first practical internal combustion engines, we discovered how to fly. Science? Maxwell wrote his equations, Einstein explained the photoelectric effect, proved atoms existed, and overthrew Newtonian physics, Max plank made physics quantum...
....by what metric wasn't it some of the most amazing years in human history?
This was all done by a population of ~1 billion people. Who actually were way less productive that people are today. If the human population dropped much below 1 billion, I might be worried. But IMHO ~1 billion seems to be a sweet spot.
and all these worries about having too many old people are way overblown. Any problem whatsoever which involves 70+ year old people is a short-term problem.