We see in Terminator: Dark Fate that John is killed in the 1990s, some years after the events of Judgment Day. So if it has been done, who is the John in Terminator: Rise of the Machines, Salvation and Genisys?
If the death of the past John has no impact on future John and his Resistance, then why is it necessary to prevent Skynet from going to the past and kill him?
Why does Skynet itself send their machines to kill young John if they know it has no effect on the future John?
And also, the prevention of Skynet's creation in the past had no effect on the future Skynet at all!