The point is that population within existential planes is in constant flux. The human plane is part of this cosmic enfold, and according to the differentiating kammic energy of the beings within our world-system (the single world, not the chiliocosm) the four lower planes, the six sensual pleasure planes, and the intermediate human plane convulse together quite rapidly compared to the entire scheme of our trichiliocosm (thousand-thousand-thousandfold system, or billionfold world-system). The human population is increasing not only because the opportunity for human life is increasing, but primarily because of a kammic influx of beings into the human state. This is supposed to occur, and especially in our world in consideration that we live in a Five-Buddha-kappa, nearing the final series of antarakappas (smallest aeons) in which the final Sammasambuddha shall arise, Metteyya. Eventually, this accumulating influx of beings is going to cause a mass expurgation of beings. This happens tremulously with the end of the mahakappa. And again the same accumulation-expurgation effect takes place after 8 mahakappas, upon which duration the entire thousandfold system is destroyed. And again it takes place after 64 mahakappas--or upon 8 destructive cycles of the thousandfold system--upon which duration the entire millionfold system is destroyed. This accumulation-expurgation effect, of course does not apply to the entire billionfold-system (the absolute extent of our material universe), in which the fourth-jhanic planes are never destroyed. The Formless realm is also never destroyed and encompasses innumerable material universes within infinite space. So, although there is indeed a reception of beings from the greater world-systems, which do ascend from other worlds, there is in fact no influx of human beings from other worlds during the last asankheyyakappa of a mahakappa. This influx (and correspondingly consequent expurgation) primarily consists of beings of our single world (the eleven sensual planes and first three brahma planes, up to Mahabrahma).
If it would happen that human beings are exterminated on this world--as you proposed in your original question--then that would be the accumulative effect of kammic influx into this plane. The cultivation of kamma cannot be wasted, it would simply mean that no one is to be reborn in the human plane. It is a natural tendency of the cosmos. If our species were to become extinct then there is virtually no opportunity for a revolution in time for the destruction of the world. The ascension of beings into higher realms would begin to take place and there wouldn't be sufficient kamma for human revolution.
This is all according to cosmic cycles due to the kamma of descending (forming world) and ascending (dissolving world) beings. According to the Buddha, there is NOT going to be a mass expurgation of human beings because Metteyya is destined to arise before the destruction of the world. In summation, the question has been rendered absolutely moot.
Sometimes no Buddhas arise in the world. Sometimes they do. When it happens, it is for the welfare and happiness of men, out of compassion for all creatures. For a long, long time he has been working to become a Buddha. He met other Buddhas along the way. And after his long striving he attains his final life, yet not without showing everyone else how to get there.