consider detectability as a severity benchmark

[kristovatlas]

users who detect that an attack has taken place are more likely to take preventative measures to avoid them in the future. this arguably comes into play for blockchain and network observers in 2 ways:

1. blockchain observation is silent, while active network observers (eg P2P Sybil attackers) are not. 2. There are arguably better open source / free tools available to point out privacy mistakes on the blockchain than there are for network-based problems, and so there is more demand for blockchain-based countermeasures.