I can think of a few other examples that critics use, like illicit purchase on Silk Road, money laundering, or anonymous funding of evil (terrorism or organized crime).

It would be awesome to actually try and quantify what the effect might be if decentralized systems are in place to facilitate them.

All of these things happen already very successfully within our old world economy. So to take your argument a step further, is there any way to calculate the potential increase in this type of criminal activity?