My point is that it is likely they will mine for other metals and materials overtime. As long as you can amass materials in space and keep it in geosynchronous

My point is that it is likely they will mine for other metals and materials overtime. As long as you can amass materials in space and keep it in geosynchronous orbit you can wait until future people get to space and can build spacecraft with them. To your point, water may be the best thing to mine but I wouldn't invest my time there specifically. I would get whatever material I could as easily as possible since I would not know what future missions/propulsion systems would be. Further, if you found an abundance of something with even a low energy/weight ratio or no known way to use it for energy it is entirely possible that someone on earth will figure it out, buy it from you and use it to travel. The cool thing is that whatever you get could be traded on some type of commodities/futures market with delivery in space which means you could theoretically sell it right after you mine it even if no one takes control or uses the material for 20 years. While the author assumes they will mine asteroids, I think it is just as likely they will build something to pick up and consolidate space junk. That will probably be the most profitable venture and has the dual property that it will prevent damage to other vehicles once pulled from orbit.