If you think about it he shouldn't exist anymore since his parents jumped to 2017 without giving birth to him. What T-John seems to think is that they all exist outside of time, from some alternate timeline and events no longer effect them.I like how they had 30 odd years to stop Genisys and decided to go forward in time to give themselves only a few hours.
They didn't have 30 years, John Connor was sent back to 2014 so they really only had three years. and the only way they knew to go to 2017 was Kyle's memory of events that hadn't happened yet.
I'd still like to know how John connor expected to live if he killed off his parents. I also suspect that it was an alternate John Connor who sent Pops back to 1973.
That is the belief of the producers and writer according to interviews. In the Terminator Franchise once you time jump you are no longer beholden to the events that happened in your reality. You are in an entirely different timeframe.
Think of it this way. In B2TF Marty breaking up his parents led to him gradually disappearing. In Terminator this would not happen. John would not vanish if he'd already jumped back in time. He could kill them both without repercussion to him. Only that the timeline he'd entered John Connor would never be born.