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I would also point out that there was a chapter in Collapse by Jared Diamond specifically on how the natives of Easter Island killed themselves trying to build those Moai, as well.
I know. Your point? They built those moai even though they were consuming wood and manpower that, pragmatically speaking, probably would have been best spent elsewhere. Hence, you can tell that rich, powerful people value things that are highly effort-consuming.
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[sarcasm]Yes, a secret. Is that why nobles will pay so much to show it off?[/sarcasm]
A secret that can be learned in worldgen. Like necromancy, but without the required divine intervention. Best part? We'd be able to mod in our own reaction-secrets.
On a more serious note, you have only put evidence up for the possibility that platinum should be valuable; there still remains substantial reason why it shouldn't be so.
The lack of malleability? Increases the effort put into it, and therefore the value some people would see in it. The lack of luster? ...Well, okay, less asthetic from many points of view, but not neccisarily all. And, once again, sometimes people spend money on silly, expensive things
because of the expense in obtaining and/or maintaining them. For some modern examples, how about mansions (how much space can one person need?), or private jets (unless you're flying a couple dozen people with you)? Those are just the ones that come to mind immediately.