Those Bitcoin mining costs are the non speculative, translated value.
No, the costs are the costs. The value comes from the fact that there's a demand. If there's no demand for gold, it doesn't matter how much it costs you to extract it from the ground: It's simply valueless.
However, in Bitcoin there's something different. The costs are somewhat connected with the valuation, because of the
difficulty, which is a parameter that does not exist in any other asset. If the demand rises by a lot, it brings more competition into mining which makes it, in consequence, more expensive to mine 1 BTC. Oppositely if the demand drops by a lot.
The canon is the canon, though. If there's no demand, there's no market value, despite of the costs.
You are confusing value with demand. Obviously depending on the situation, bread might be more valuable than 100 Bitcoin or 100 kg of Gold. But thats an apocalyptic scenario which most likely will not come true anytime soon. Hopefully.
But my point stands. Anything can drop or rise in value but there will always be inherent value in work and time. The work in this case being electricity, equipment and so on and the time being the amount of time you spend mining to create a single Bitcoin.
But then again your logic would also work for anything, given the circumstance. If aliens enslave all humankind and put in a system where money is based on some arbitrary resource, that resource will gain a speculative value due to its demand. Or if we discover gold cures cancer, price of gold goes up.
But the minimum value is not raised by demand. Its raised in the minimum effort put into it.