Every zero-sum adversarial system would be a lot more efficient if they accepted the judgement of a third party instead. How much energy could we save if we dismantled the court system, elections, and pro sports?

I think DAG coins could be a lot more efficient, but I'm not totally solid intellectually on how they achieve consensus on conflicting spends so I'm not sure it will work as a general solution. The only one I've used, nano, has I believe no mining reward, and I think consensus is achieved based on a voting process weighted by proof-of-stake among online nodes -- the incentive for participating in the system is that the system has value for you in other ways. I'm not closely following ETH PoS research, but it will be interesting to see what they can accomplish, it could create a huge reduction in energy consumption.