[LyndsySimon]

At some point, "extracting" resources from landfills becomes economically viable.

This is the beauty of the market - interests are balanced against one another and an equilibrium is reached. If more people want cars than can be built, prices go up and demand goes down. Someone out there finds a way to make cars more cheaply, which reduces per-unit resources costs, allowing more people access to them.

Also, overpopulation is never going to be an issue in the sense that you're considering here. Population growth is inversely correlated to economic success, to the point that developed nations have reproduction rates below that of replacement. As people are raised out of poverty the world's population growth will slow, and if we are successful at eliminating poverty, reverse.