The biggest issue with proof of work crypto currencies is the "inefficiency" is the goal in and of itself. If someone makes a plane 10x more efficient tomorrow

The biggest issue with proof of work crypto currencies is the "inefficiency" is the goal in and of itself. If someone makes a plane 10x more efficient tomorrow every airline is going to switch. If someone makes a bitcoin miner that is 10x more efficient they will need to 10x the difficulty to mine or it will become too cheap to attack the network. Proof of work, unlikely almost anything else, is fundamentally opposed to efficiency gains.