I think that some day the humanity will grow too much and will get tired of overpopulation (I'd hope 30 billion would be the number, but if the humanity is arrogant, they would push it up to 100 billion or more), some people will just die because the planet can't provide enough resources, some rich people will buy other peoples live and kill them (give them a ton of money only if they are willing to die soon) and the world population will agree in not growing anymore but enjoying and preserving the planet as it is, maybe staying in a billion population. After that the world will start to restore and men will have to do nothing because machines are already doing everything, and the only jobs the humanity will have are going to be science and entertainment (incluiding arts and sports)... humanity will no longer grow not because they can't, but because they don't want.