I'm perplexed by this statement as the journey would be the exact opposite of what you purport. Are you saying that, unless we can travel to someplace that is very hospitable to life, that we shouldn't bother? How are we ever going to develop the technology to go to such a theoretical place if we're not working on the technology already and constantly iterating on it?
Let's say we're going to be hit with an asteroid that will kill all / most life on Earth. There is no way we can stop it. Having colonies on other planets and moons within our solar system means all of those people, in their self sustaining habitats, will live through the extinction event. Humans would not go extinct.
> If we can manage to survive on Mars, certainly we can manage to survive on the most life-friendly planet known to man.
Are you saying there is no such thing as an extinct level event for humans simply because we can live on Mars? I'm not sure what the point is that you're trying to convey here.