Although "the value of a good tends to its production cost" is not wrong in many cases you cannot reverse the argument. Production cost compromises a lot more than electricity. Wasting energy doesn't produce value. And Bitcoin mining means wasting a lot of energy (to secure the network and to distribute coins) as soon as we observe that the same result can be generated using other (less costly) methods. It is possible that PoS can fill the gap here (and that's probably also the reason why loads of Legendary members and even Bitcoin developers spread FUD about PoS).