It doesn't assume. Hence 'substitute'. long run refers to the amount of time for prices to adapt, not a short run dependence on one thing.
Hence change in factor inputs.
An example of a change in factor inputs was, for example, whale oil which was a big deal for lighting systems in the 19th century. But as cheap whales were running out, so whale oil, thought to be so important for many and which there were technical improvements in sourcing and usage, was shifted away from.
As will be the case for copper, as it's a derived demand. The derived demand will be more price sensitive than the demand for the end service (lighting in the case of whales, or Open AI for copper). Not happening with copper in the short run will only make the long run change happen even (as time passes) faster.
Hence change in factor inputs.
An example of a change in factor inputs was, for example, whale oil which was a big deal for lighting systems in the 19th century. But as cheap whales were running out, so whale oil, thought to be so important for many and which there were technical improvements in sourcing and usage, was shifted away from.
As will be the case for copper, as it's a derived demand. The derived demand will be more price sensitive than the demand for the end service (lighting in the case of whales, or Open AI for copper). Not happening with copper in the short run will only make the long run change happen even (as time passes) faster.