Different Rationale
I agree with your conclusion, but the basis is incomplete IMO. I agree with the getting the eggs out of one basket reason, but also see benefit to the home world. An interplanetary civilization is far more likely to develop the propulsion technology and power capable of diverting even large comets than a planet bound one. I also see terraforming tehnology developed for Mars as potentially useful in reclaiming ruined land areas on Earth. I see colonizing Mars as a means to help protect and nurture Earth as well as humanity. I am firmly of the belief that the Earth is a preciously rare planet that deserves protection even if humanity grows beyond it.
Regards,
Mark