I didn’t mean to imply that there is a conservation of wealth. I meant that there is no exchange with participants outside of the population of Earth; and at any given time, if you want people to be able to spend x, they must also be able to earn x. And if you have an economy that works with a population of n, then in principle the same should be possible with a population of (say) 2n, because you can just imagine having the first one twice independently side-by-side. The only hard limit is when you exceed Earth’s resources, hence overpopulation. But we’re not there yet, and according to projections we also won’t reach it this century. The reason for our present difficulties is not that there are too many people.