Miller's World isn't actually (on the face of it) that bad an option. It has free water, a breathable atmosphere, organics compounds and hydrocarbons in abundance. You may also wish to note that the "star" that Miller orbits is in fact a gigantic black hole with a lifespan of trillions of years rather than the run-of-the-mill hydrogen/helium stars we're used to.
“Look,” he said. “Dr. Mann’s data looks promising, but we won’t get there for months. Edmunds’ is even further. Miller hasn’t sent much, but what she has sent is promising—water, organics.”
and
“Look at Miller’s world—hydrocarbons, organics, yes.
It has everything, in fact, that's needed for Plan B to go into operation, regardless of the fact that the Earth would be dead and buried before even the first generation is of age.