To a certain extent, that's life. You may be able to save humanity from Skynet, but it's naive to think that single act would save humanity for all time. It's a never-ending project. To use the Biblical analogy Terminator is so fond of, the day of the Lord comes as a thief in the night. Two men stand in a field, one will be taken, one will be left behind. People will tell you it's about the end of the world, but it's really about mortality and the mystery of death. Nobody knows when, nobody knows how, but it's coming, and the best you can do is try and keep it from coming today.
I saw it once pointed out that this is kind of endemic to science fiction and fantasy in a way that'd be absurd in other genres. "The detective broke up the drug cartel, and that was the end of all crime forever." But you show that a sci-fi hero "only" ensured twenty or thirty years of peace and prosperity, and you're denigrating the original story and dismissing the old heroes and so on and so forth.