That seems to assume a perfectly-efficient currency, one whose value is always directly proportional to the work invested. But there's no evidence that BitCoin

That seems to assume a perfectly-efficient currency, one whose value is always directly proportional to the work invested. But there's no evidence that BitCoin actually works that way. I have no guarantee that I would spend the same cost to mine one Big Mac's-worth of BC as I would to buy a Big Mac, because the price fluctuates wildly due to all sorts of other factors such as hoarding, worries about forking, etc etc.