It's the same situation as gold and gold mining. The marginal cost of gold mining tends to stay near the price of gold. Gold mining is a waste, but that waste is far less than the utility of having gold available as a medium of exchange.
I think the case will be the same for Bitcoin. The utility of the exchanges made possible by Bitcoin will far exceed the cost of electricity used. Therefore, not having Bitcoin would be the net waste.
I agree with nearly everything you said, but I disagree, fundamentally, with the bolded. Gold mining is not a waste of energy. It is the opposite of waste, it is the measure of value people place in the 'utility of having gold as a medium of exchange' or a store of wealth, jewelry around their body parts or connectors on their home theater system. If there was no demand for gold, the price would be zero. Hence it is not a 'waste,' by definition.
I object strenuously to this idea, promulgated by the Monetarists, that the production of money is wasted capital which could be spent on other wealth-building projects. It's a short-sighted argument which does not fully encompass the value we place in our money.
It's semantics, yes, but the negative connotations associated with the word "waste" is a tool of the money masters designed to confuse us and elevate their bankrupt system.
Ta,