Star Trek Time Travel Rules: Multiverse. There exists an infinite amount of universes. Travelling back in time doesn't change the history of the original universe. It creates a new timeline in a different universe.
It's best if they just reboot it with a clean slate and move away from Conner and Reese as the center point or make a soft reboot like Mad Max Fury Road which is a sequel but also a reboot with a new actor as Max.
The only movie that makes sense is Cameron making his terminator 3 which would be a future war movie and John Connor winning the war against the machines and ignore 3 - 5.