Kidding aside, I have zero clue what the actual adoption rate is for Monero in the seedy underworld of common criminality, so I can't really challenge anyone on this argument.
I can say if I were a criminal, I'd want to keep it as low tech as possible. Given how sophisticated computer forensics is, I don't think I'd be able to keep everything "clean", digitally speaking.
For example, many professional bartenders commit tax fraud every day by not reporting cash tips. This is directly facilitated by paper currencies.
If tips were given in Monero, the tax fraud would be even easier.
EU (or Germany, sorry I forgot) just passed laws that production of all goods should be traceable from beginning to end.
They are working on it. Nobody shall be able to opt out of the system.
In light of this - I think it's rational to remove paper currency from circulation and to move to a digital system where every transaction is tracked to a real person. This could be as simple as using existing banking structures, or maybe some sort of cryptographically-secure, immutable ledger of some sort, tying every transaction back to a real person. Almost like a centralized, managed "block chain".
/s