Some people, like Keith Henson, argue that we've blown the thermodynamic opportunity to get off planet because we've already squandered the best quality fossil fuels.

This seems very strange. First, most of our rockets use hydrogen and oxygen. One doesn't directly need to use fossil fuels. Second, there's still a massive amount of energy available in terms of fossil fuels, it just involves lower energy return on energy investment. Third of all, there are many other sources of energy, with nuclear power being one of the more obvious, but massive amounts of solar and wind also being available. Fourth of all, rockets are comparatively inefficient in general since one needs to move most of the fuel itself. Launch loops and space elevators are both obvious substitutes that are more energy efficient once they are off the ground. Do you know where Henson has made this argument and what his reasoning was in more detail?