Novel and Screenplay
Jack Finney's novel ends with the extraterrestrials leaving earth after they find many humans offering too much resistance, despite having almost no reasonable chance against the invaders. Also, the "pod people" have a life span of no more than 5 years. As a result, 5 years after taking over the last human being, the invaders would have to look for a new planet with new life forms as hosts – leaving behind a depopulated earth.
Read more about this topic: Invasion Of The Body Snatchers