What limits the length of the lever? The agricultural lever is already crazy long, the manufacturing lever, same. We could be doing the same with less, not more with the same.
Depends on where in the world you're looking. In India, something like 50% of the population works in agriculture. At the scale of India's population that's a significant fraction of the population of the planet, it's more than twice the population of the entire US.
When do we stop?