Nitpick: It wouldn't have to be commercial use of nuclear energy. Even if we're limited to human institutions, it could be governmental use, and I have a notion that religion might be the best sort of institution for getting people off the planet. Religions have a potential for big, long term projects that don't make practical sense.

Thanks for looking into the question of planetary mass and getting off the planet-- once the question occurred to me, it exploded into a lot of additional questions, and we haven't even gotten to whether planetary mass might have an effect on the evolution of life.