One TV Tropes User had this to say about John's death:
tvtropes.org
"People were complaining about that in Genisys too. But, like, here's the thing. John isn't the main character. He never was. At least, not until T3 and T4 tried to push him as the new protagonist.
John is a
Living MacGuffin. He's not even a character in T1; he exists strictly through reputation. He's the Terminator's motivation for trying to kill Sarah and Kyle Reese's motivation for protecting her. If Sarah survives, her son wins the future war and Skynet is toast. His existence is the ball being fought over by Kyle and the Terminator.
Then T2 comes along and the T-800 is after John. But he's
still a
Living MacGuffin. He's the victory condition; if the T-1000 kills John, Skynet wins, but if Sarah and the T-800 protect John, humanity wins.
Both films assure us that so long as John lives to see Judgment Day, humanity wins. But T2 prevented Judgment Day, which has been a thorn in the side of every attempt to sequel the films since. John, by all rights, should just be some guy in a post-T2 world.
Even if Skynet does wind up coming into existence years down the road like T3 postulated, the John Connor that led the war in T1 doesn't exist anymore. That future rendition of John who eats machines and shoots lightning from his dick? Sarah killed him alongside Skynet and every other event that would have happened after Judgment Day.
Our John Connor isn't Thunder Dick John Connor. He's just Some Guy John Connor now, with nothing to offer the future beyond vague promises of an important destiny from a
different future robot war.
That's actually one of the few things there is to like about T3 and T4. They make a plot point around the fact that Thunder Dick John Connor doesn't exist anymore. In stepping into the role of protagonist, Some Guy John Connor has to wrestle with living in his own shadow. He'll
never be Thunder Dick. The events that created Thunder Dick are gone forever. Now there's a whole bunch of important people to the war effort that Skynet needs to kill, 'cause humanity's still winning the war but Some Guy John Connor can't just walk into Machine HQ and rub his balls in Skynet's face.
But those films are terrible and no one likes them, so they don't count. For Dark Fate, Cameron's excised them from continuity, which means he's right back where T3 was: John Connor doesn't matter anymore because the events leading to Thunder Dick slapping down Skynet were averted. So he's not a main character AND he's not a
Living MacGuffin. In terms of plot relevance, where does that leave him? T3 and T4 tried to make him a main character and those movies sucked, so I guess the only other answer is "expendable for drama".
Thoughts?