One thing to consider is that the sciene problems we have to solve now are much harder than what we had to deal with 200 years ago at 1 billion population. You

One thing to consider is that the sciene problems we have to solve now are much harder than what we had to deal with 200 years ago at 1 billion population. You are correct that what matters is mostly the population living in soceities organized well enough for their inhabitants to have better prospects. But sadly becoming a country with "better prospects" is not a simple thing. In fact, population decreasing in rich western countries, has a rather deleterious effect on the poor countries. I being from a third world country see everyday how people who could help to fix my country end up emigrating because rich countries need more people. So it's not at all clear whether much of the world getting better prospects will compensate for population decreasing, or much of the world will get worse prospects because of population decreasing.