Maintaining a decentralised ledger is necessarily inefficient compared to a centralised equivalent. The more people that participate and make the network less susceptible to attack, the more energy is spent. However, making a problem arbitrarily difficult in order to prevent (in bitcoin's case) a peta hash network from doing it's job too well (consistently finding blocks in less than 10 minutes) is only one solution to the problem - there could be other solutions...
btw Ethereum was the first place I read about a memory hard proof of work scheme but apparently that was dropped. Can anyone confirm this?
