Also, trying to kill Sarah or her forebears raises the risks of greater and greater mistakes. Maybe her grandmother is caring for another child the day the Terminator hits. Maybe the actual Sarah is kidnapped by a crazed childless couple, and years after the Terminator hits, finds out who she really is. She has a messed-up life, and falls in with the wrong friends, one of whom shows her how to make pipe-bombs. She cleans up her act just in time to find a menial but stable job as a waitress... Also, lives can realign. Sarah's grandma and granpa die in the 40's, but at the funeral, his brother and her cousin meet up. By the way, dark family secret : Her cousin is actually a half-sib. Wipe out the clan? Too bad about the brother forced to change his name when he took a job the family didn't approve of, and whose name was wiped from all family records. His family gone, he then resumes his name in their memory. Remembering the slaughter, he makes sure all his heirs get some form of military-style training... It may not have worked so far, but by keeping its focus on the verifiable Sarah and John, Skynet avoids splitting the timeline past its comfort-zone of computation. Even a super-computer can only account for so much, when nigh-infinite parameters are at play.