Different risks: using up all resources (running out of fossil fuels) vs an instantaneous mistake/disaster (global fallout some some weapon, or an asteroid, or

Different risks: using up all resources (running out of fossil fuels) vs an instantaneous mistake/disaster (global fallout some some weapon, or an asteroid, or freak solar flare, or something like that). Colonizing another planet protects against the latter and probably shouldn't be dismissed simply because it is not yet a solution to the former.