If you wanted to depopulate the world, and you were in power the last thing you would want is to kill people by starvation. Hungry people start riots and revolutions. If oil is the limiting factor in population growth and maintenance, they already know how much of it is left and can calculate what size population it can sustain in the future. They won't let us starve. They will make us infertile with, oh I don't know, a vaccine.