Oh thanks, now I see it, these almost-there cases looking somewhat holywoodish, like the villain obligatory should pronounce lingering monologue before actually killing his victim, and thanks God hero appears in last moment.

Okay, Skynet which won't instantly get rid of humanity is improbable, if really superintelligent, and if it has this goal.

We can easily imagine many kinds of possible catastrophe which can happen, but we are not equally good at producing heavenlike utopian views, but this is an evidence only about our lack of imagination