Did you like read the thread? Point 6 in cletus' opening argument.
Rare materials will not make you as much money as you can make now because they will become not rare. So you have to make something which only can be made in space or you have to play the De Beers game, be an evil fucker and spend all eternity in hell if it existed.
Platinum is used in a lot of stuff. Catalytic convertors, electronics, but perhaps most importantly in fuel cells. Usage is going to increase dramatically, meaning people will need this stuff. Getting something in space down to Earth surface is not as expensive as escaping Earth gravity.
You really think platinum is going to drop from $50,000 per kg to something like $1,000 per kg before they've made their money back?
Rare materials will not make you as much money as you can make now because they will become not rare. So you have to make something which only can be made in space or you have to play the De Beers game, be an evil fucker and spend all eternity in hell if it existed.