It probably depends on the metric you're trying to optimize. I was curious about that as well, but I was thinking in terms of the maximum number of humans the earth can support. We certainly haven't hit that yet, we have large swaths of land allocated to relatively inefficient means of producing food (i.e. cattle).

Also, don't we need some level of population growth for Social Security to function correctly? We could replace Social Security, but it would be difficult and expensive.