AsI recall, they implied pretty clearly in the second and third movies that previous time travel did in fact change outcomes - at least in small ways. There's no one single self-consistent timeline. There's just one timeline that you, J Random Observer, happen to be in. Which can and does interact with previous timelines, and probably interacts with future timelines as well. The struggle then is to change things in larger ways, since it appeared that some events the rise of Skynet) still had a scary tendency to repeat themselves. Like a main theme repeated in a different key.
(At this point I feel obliged to point out that Terminator's current TV series is, while flawed, still much, much better than the Planet or the Apes TV series. I'm only drawing a parallel between the storylines; I'm not saying they're comparable in quality.)