Mining is always 100% efficient. If you don't understand that, then you don't understand how a blockchain works. The purpose of mining is specifically to waste

Mining is always 100% efficient. If you don't understand that, then you don't understand how a blockchain works. The purpose of mining is specifically to waste something of value. Without destroying value, there is nothing to keep the network anchored to 'truth'. How much energy is needed to be burned? Precisely as much as is needed to combat 'spam' (both in the form of speculation and block-space consumption). The only 'alternative' that has been pitched is the ironically named Proof of Stake. PoS has many, many problems (Andrew Poelstra itemizes many of them) not least of which is that they will degenerate into Work-based attacks/mining-strategies. Thus far, miners have been reluctant to switch to an alternate chain. It is probably a very tall claim that this will ever happen at this point. SMTP was/is a terrible protocol, but the advantages of the network effect sustained its success. This effect is particularly pronounced in Bitcoin, mostly due to the way that blockchains work. The longest chain will almost always secure the most 'faith'.