2. WHY HUMANS MUST GROW INTO SPACE

My reasoning is simple: If we don't get into space, then the human race will inevitably die, just like the dinosaurs. We will be killed by an asteroid, or a comet, or the death of the sun. It's always the tendency of life to grow and spread, and thereby minimize the risk of being annihilated by a local disaster. Evolution gave humans minds so that we can take that process to the next level, and ensure the survival of life (not only our own, but also our fellow creatures).
Also, although I am in favor of a limited powerdown, I believe people like Heinberg are making a big tactical error when they bash technology and say "we'll never get to space". They are trying to sell people on the idea that we must retreat back to the earth, and the past, and (as noted above) eventually death. That view is too hard to sell. You've got to have a light at the end of the tunnel to make the hard times of powerdown worth doing. So I like to think of it like metamorphosis: we have to powerdown and conserve our energy now, so that we have enough to manage the transition to clean, unlimited space energy -- i.e. the butterfly we should become.