Yet another point: Suppose we’ve got a civilization advanced enough to spread over the globe, including both the northern and southern hemispheres. Just what, exactly, would be capable of wiping them all out, everywhere? Even in the worst-case WWIII scenarios that kept us up at nights in the height of the Cold War, New Zealand would have gotten out pretty much unscathed, and Australia, Argentina, and Chile would be only mildly inconvenienced.