In the end all our eggs as in the same basket as long as the solar system, the galaxy or the universe would eventually disappear. Allowing billions of billions of human to live for the next thousands of year is quite irrelevant: nobody asked to be born, so nobody won't miss the opportunity. As for our legacy, 99.995% of us don't leave a trace meaningful enough to be remembered as individuals by our grand grand grand children.

So, OK to conquer Mars, but not at any cost because the ROI seems really low to me.