> hopefully something generally useful to society at large If it is useful then it won't be secure because it would be free to "hash". The mechanism has to be

> hopefully something generally useful to society at large If it is useful then it won't be secure because it would be free to "hash". The mechanism has to be costly. Proof-of-work is in many ways proof-of-waste. If tomorrow we find out that hashing is profitable then everyone can start hashing and be able to attack the chain for free (subsidized by the usefulness). Only the hash power above the useful level can add security.