The concern is not a disaster that necessarily destroys Earth itself - an extinction event for humans is enough. There are plenty of species that have gone exti

The concern is not a disaster that necessarily destroys Earth itself - an extinction event for humans is enough. There are plenty of species that have gone extinct before (including some close relatives) and since it's obviously never happened to us we have little idea of its shape or likelihood. The very distance that makes Mars so hard to reach would be good protection against such an unexpected event (whether it's unintended effects of scientific experiments, novel WMDs, grey goo etc.). There is no way to create a similarly isolated colony on Earth, even if we discount the problem that nobody would want to live in such a place. (As opposed to the many people, reared on SciFi, who would volunteer for Mars quite eagerly.)