Why was Kyle Reese sent back in time?
https://www.reddit.com/r/movies/comments/ereqdn/in_the_terminator_1984_why_was_kyle_reese_sent/
If you're John Connor in 2029 and you see that a Terminator has just been sent back to 1984 to kill your mother, the fact that you still exist inherently means that Skynet's plan didn't work. Even just a split second after the Terminator went through the time machine, from the perspective of people in 2029, it's already had 45 years to complete its mission.share
Had Skynet's theories related to time travel been correct and the mission been successful, John Connor would have ceased to exist at the exact moment that the Terminator went through the time machine, leaving no time for him to do anything about it, or anything at all, for that matter.