Only problem is, fiat is better at most of the "good-natured" crime too.
The only application I can even think of where cryptocurrency is actually better than fiat is ransomware. Actually I suppose any kind of ransom payment might benefit from using crypto, as there's no risky physical cash handover for police to intercept. Seems like it could be equally useful for kidnappers as ransomware hackers.
But I can't think of single non-ransom transaction where fiat isn't better than crypto. Legal or otherwise.
For example when buying illicit substances, I don't see why I would want to pay extra to make the transaction traceable and public?
Without incriminating myself too much, uh, let's just say I've seen considerable volumes of illegal business conducted with physical fiat cash, to a degree that likely would have been more difficult with crypto
Fiat money, unlike crypto, have lots of non-criminal uses.