The best argument I know against expanding into yet another frontier is expressed in the Eagles song "The Last Resort". The basic message of that song, as I understand it, is that we ruin every new frontier we conquer. Or as the closing words of the song put it, "you call some place paradise, kiss it goodbye." So maybe the universe is better off if our damage is limited to our own little planet.

Then again, that song also said, "there is no more new frontier," seemingly ignoring space, so maybe Don Henley was just being overly pessimistic.