My guess is that they will only intend to ship back the very expensive/weight minerials/metals and will leave in space materials with more weight but use in future missions.
Once you establish materials in space, power plants in space, robots in space, it is a lot easier to move things further out.
The end game to me is having separation between heavy materials in space and on earth and only transporting the lightest and most valuable between the two. (humans/gold/diamonds) etc.
With current launch costs everything in LEO is worth about as much as gold per LB ~5-10k vs 20k. Thus in the short term mining water taking it to LEO and splitting it into Oxygen and Hydrogen for fuel tanks is probably a lot more profitable than shipping Gold / Platinum back to earth.
PS: An ION drive tug boats to move stuff from LEO to geosynchronous orbit is IMO the most needed infrastructure to make space cheaper.
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.
Storage on long time frames would mean more expensive, long-term bets for the companies responsible? I'd assume that they'd already be gambling in the early stages setting this stuff up and looking for a pay-off within a reasonable time frame.
I'm not sure if it would cost much to store or not. They might be able to use carbon fiber boxes (built in space) and get it in to high orbit. Satellites can stay up there for a long time with limited/no additional expense.
Once you establish materials in space, power plants in space, robots in space, it is a lot easier to move things further out.
The end game to me is having separation between heavy materials in space and on earth and only transporting the lightest and most valuable between the two. (humans/gold/diamonds) etc.