Some say that the Demons and undead have bought so much destruction to the earth, twisting it, befouling and corrupting it that even if the forces of good heart are actually able to prevail against the darkness that they would inherit a dying world, one which would collapse in on itself. Some accounts say that people have even seen the world bleeding with black blood welling up from the deepest parts of the earth.
Others say that there are some areas where life continues despite the demonic presence. Demons soon bore with unintelligent animals and turn their attention to torment the sentient races of earth. Deer run from wolves or Balor alike and all creatures can survive. The taint they leave behind is cleansed by rain, tide and time and that even the foulest taint will leave the earth. The fight is never hopeless.
Who knows which of these is true.