John Connor’s role as Saviour of Mankind is the result of a positive feedback loop. In an earlier (now supplanted) timeline, John was just a random guy who happened to come to Skynet’s attention; maybe he was on a list of low-level Resistance recruits whose names got into Skynet’s possession somehow. Skynet sent a Terminator back in time to gather intel on a whole bunch of people, John being one of them.

John happened to notice that this guy was acting funny, and it stuck in his mind. Years later, he recognised the same model of Terminator trying to get into a Resistance base. He put two and two together, raised the alarm, and the Terminator was destroyed – but not before uploading an image of John to Skynet. As a result, Skynet sent a Terminator back in time to deal with John specifically. It failed, somehow, but it led to young John learning even more about Terminators, which made him more useful to the Resistance, which led to Skynet targeting his mother, which led to Sarah turning badass and raising John as a figher, which… and so on.

In other words, Skynet created its own worst enemy by mailing a bunch of its own tech back in time for him to study and prepare for.

* John’s role as the saviour of mankind was set up by Skynet. They were having trouble wiping out a Resistance with a distributed command structure, so they sent agents back in time to raise the profile of one leader, who would then be a single point of failure who they could kill easily.

* John is a scapegoat. The Resistance found out about Skynet’s time travel capabilities, so they spread rumours about one great leader (who in reality is a random nobody, is already dead, or doesn’t exist at all), in the hope that Skynet will focus their efforts on him instead of the real leaders.

* Kyle Reese was a robot with functioning reproductive organs, sent to the past to impregnate Sarah Connor with a hybrid child who is programmed to (a) be a charismatic leader and (b) rebel and kill the humans as soon as they put him in charge.

* Skynet won the war before they developed time travel. The missions to the past are either (a) scientific experiments to study causality, (b) reruns of the past to see how the war could have happened differently, or (c) out of boredom.

* The amount of power needed to travel back in time is related to the length of the jump, so going back to the recent past is quite easy. In the year or so leading up to the invention of time travel, entire armies regularly appear out of nowhere.

* As John Connor grows up, it becomes increasingly obvious that he looks like Arnold Schwarzenegger. It’s many years before he finds out what the link is.

* One day, John runs into a (demonstrably human) guy who looks like Robert Patrick. The poor guy has no idea why John freaks out at him.

Actually, a better way to do the film might have been to leave John out entirely, and make it a non-mainline Animatrix kind of story. We’d be more forgiving of a non-formula Terminator if it’s explicitly about what else is happening in the universe to people who aren’t named Connor.