Presumably because somebody pointed to an asteroid somewhere and calculated that it contains a volume of platinum/gold/unobtanium that would be worth a hundred zillion kajillion trillion dollars, without taking into account that if you actually brought that much of a given precious metal into the market the price would crash to zero and we'd all be drinking Coke out of disposable platinum cans.

On the one hand, this is technically true. On the other, this is effectively because the gains to such a large windfall cannot be captured by a single entity: Aluminium becoming so cheap as to become a nearly disposable material is a massively _good_ thing.

It's not inconceivable that having platinum become plentiful might also be such a boon.

Sure, but using aluminum as an example, if we take the modern production of aluminum (as a proxy for underlying demand) and multiply by the price of aluminum in the 1800s (when it was so incredibly precious that it was chosen to cap the Washington Monument), we would expect the modern aluminum industry to have revenues of around $100 trillion, or about 1000x what it actually has. The point is just that one cannot assume a resource extraction industry's future profitability without taking into consideration how increased supply will also lower the price of the product.

How much economic benefit / efficiency did aluminium bring vs had it not existed?

Impossible to tell, but a ton more than what aluminium industry pulls in.

Isn't that the goal? If we can get rid of scarcity in materials and energy then everyone's living standards can go up. If we can move metal smelting off world, pollution goes down at the same time. Am I ignorant to think that would be a big positive for human kind?

I'm not saying it's not a potential positive for society, rather I'm saying that the economic calculations shown so far are exceedingly simplistic and merely extrapolate from the current market price of these metals, so we cannot use these numbers to conclude that this will be the most profitable endeavor in human history.

It will costs trillions to bootstrap that industry in space. It would cost mere billions to make that industry on Earth cleaner. The Earth is fucking gigantic. It's literally filled with raw materials. Access to those materials is downright cheap compared to attempting to access the same material in space.

at the very least people would stop stealing catalytic converters, that's a win