Each "future" that the humans and terminators are leaving in an attempt to change the past, is a separate "alternate future" that is from a different timeline than the "new alternate past" that these future entities are entering. This would "have to be true" in order to prevent numerous "temporal paradoxes." <PERSON> would not have sent a <PERSON> back in time where they "knew" that their mission to kill <PERSON> failed. Therefore, there has to be two entirely separate parallel realities. Someone from Timeline A can never change the events that led to the future Timeline A. All that their time travel does is change the destiny of themselves and other people and terminators in a separate Timeline B. In effect, both Skynet and the resistance are not "changing" their own fate, but rather the fate of their alternate selves in the new timeline that will result from them having entered an alternate past.
In Timeline B, even if <PERSON> or a young <PERSON> are killed, the alternate <PERSON> from Timeline A still goes on to lead the resistance that inevitably smashes Skynet's grid. As <PERSON> said [about Timeline A] "Their defense grid was smashed. We'd won. Taking out <PERSON> then would make no difference. Skynet had to wipe out his entire existence!" It was completely pointless for Skynet to send a terminator back in time to kill <PERSON> because they would only be killing <PERSON> in an alternate reality [Timeline B], not their own reality, Timeline A. 516
Do you see the paradox? Had <PERSON> and/or <PERSON> been killed decades before the 2029 of Timeline A, then no terminator from Timeline A would have ever been sent back to kill <PERSON>. Clearly, any way you cut it, <PERSON> from Timeline A survives and any past that either <PERSON> or a terminator would travel to upon leaving "their" 2029 would be in a different timeline than the one they came from. It's not that <PERSON> and the T-800 could not change "the future" by altering the past, it's that they could only change the alternate Timeline B that they had entered upon leaving Timeline A. Meanwhile back in the 2029 of Timeline A, <PERSON> and the rest of the resistance will never see "their" <PERSON> again. Instead, a younger <PERSON> will be born in the early 2000s of Timeline B unless something happens to alter this event. Even if neither <PERSON> or <PERSON> are ever born in Timeline B, this would not change the respective creations of <PERSON> or <PERSON> in Timeline A. All that would happen is that <PERSON> from Timeline A would live the rest of his life in Timeline B. At the same time, nothing that anyone does in Timeline B can change what either happened or "will happen" in Timeline A. The adult <PERSON> from Timeline A who is now trapped in the 1984 of Timeline B may or may not be able to stop the Judgment Day in Timeline B, but no matter what, Timeline A will remain unaltered. The people alive in the 2029 of Timeline A will continue to exist in the ruins that resulted from the the Judgment Day of Timeline A. Timeline B from 1984 onward has a lot more malleability as far as <PERSON> and <PERSON> being able to prevent their own timeline's Judgment Day, but anything they change would only alter the future of Timeline B, not Timeline A.
In effect, as a result of the first T-800 and <PERSON> going back in time, they were only able to alter the future of a "different timeline" not their own. 950