A "plan B" for the future of the species, like colonization of Mars, doesn't benefit the individuals sent there. Living on Mars as a colonist would be crap; it's more inhospitable than Antarctica and it's much harder to get people and supplies there and back. All it achieves is that the human species continues to exist. Being a Martian colonist and hearing news that Earth suffered an extinction-level event like an asteroid collision would not be fun.
And another planet is a bad "plan B" anyway; planets are a terrible space habitat, they just happen to be the only self-sustaining ones that currently exist. We shouldn't live at the bottom of a gravity well if we can help it.