Drug dealing and other bad stuff are there for centuries. Even with fiat currency, it's very hard to detect them because of there ties with government officials. Bitcoin price is volatile yesterday it was 7100 and now it's 6800. I don't think its a good idea to trade drugs for such volatile currency.
That's true, crimes are for centuries. Government can do almost nothing with that. Nowadays, when technology (including cryptocurrencies) developed so fast, crimes has grown. New laws and 5AMLD aren't able to stop it completely, just prevent it in some way. I think it's quite useful and might be helpful in some cases.