According to the classical, single timeline/universe view, it is impossible -- unless Cyberdyne Systems develops Skynet technology independently of any help from the future.
Assuming the existence of a multiverse of timelines, this situation can also be explained. In the universe in which we see the Terminator technology destroyed, Skynet will never exist. However, there must exist at least one timeline/universe in which Skynet technology is developed. This may occur due to: a) Cyberdyne independently creating the technology, or b) Terminator remains originating from yet another timeline are left behind. Thus, the movies must chronicle two different universes: one with the hellish future dominated by Skynet, the other is the one saved by Sarah and John.