Here's an analogy or thought experiment that was written by "Weightless" in a post in the Orbiter Forum on January 7. Of course this doesn't prove anything either, any more than the 1903 Wright Flyer proved that 747's would be carrying hundreds of people from New York to Tokyo seventy-some years later. But I found it thought provoking and wanted to share it (this is just the first part of his full comment - he goes on to relate this to the possible benefits to humanity of investing in space development):
Imagine that you could go back in time to the year 1930. World economies were starting to depend on oil, but oil was scarce (the great Saudi fields would not be found for another 20 years). So imagine that you went back in time and you told someone that they should research ways to drill for oil in the North Atlantic.
Undoubtedly, they would laugh at you. I mean, just think about it. The North Atlantic, one of the roughest seas in the world, full of icebergs in the winter months, not to mention the fact that it's deep. From their perspective, it would be impossible to actually drill for oil out there. It would be the stuff of science fiction.
So, if you went back in time and suggested it, the objection you would get would be this: "even if it was technically possible to drill for oil in the North Atlantic, it will never be economically viable - you will spend more energy (and money) getting the oil than what the oil could possibly be worth."