A large impact could melt the crust of Earth to the mantle. Is that bad enough for you? There is no reason to think that impactors will be limited to mere Chicxulub size.
Distance to Mars per se is not the issue. It's the fuel requirement vs the transit time that matter. If human missions were sent at optimal planetary alignments and with constant ion engine acceleration, the transit time could be reduced to a few months. That reduces the need for on board life support, reduces radiation exposure, reduces health effects from zero gravity (since there would be "gravity" from the constant acceleration, and makes survival more probable.
What humans would do when they got there is what we are supposed to be doing here.... building civilization. The mission is not about science but about survival. That's why it's worth the blood and treasure.
You assume that our Earth civilization will still be able to support a Mars colonization mission in 5 centuries. I don't know when the pinnacle of technological civilization will occur. The sooner we get about the business of establishing a secondary self-sustaining outpost, the safer we will be.... from asteroids, runaway AI, grey goo, nuclear war, weaponized ebola, etc, etc.
I agree with you about the unsuitability of "inspiration" as a motive for colonization. I said that in my original post.