There is a dichotomy in real cost/benefit ratio for PoW systems as spam control between real users and attackers. Attackers have more to gain from accepting the

There is a dichotomy in real cost/benefit ratio for PoW systems as spam control between real users and attackers. Attackers have more to gain from accepting the PoW penalty than users do. There is no difficulty of PoW that would both deter attackers with a financial motive and be reasonable for average users.