The basic idea is that when our human population grows large enough, we will run out of resources, most important of which is food. Have you considered though, that as our population grows, humans themselves will become a food source to other humans?
Of course, our population may be growing too fast for our beliefs about the “sanctity of human life” to change enough to tolerate cannibalism. But what do you think about the possibility? Might we simply become a cannibalistic society and continue on our normal course?