The crimes take place in the physical world anyway, and that is where law enforcement has to be.
Standard law enforcement methods are not hampered by bitcoin.
If you order drugs there is a box at the postal office getting picked up.
If there is a hit , there is a person with a gun and a person with a motive.
If somebody buys plutonium there is a supplier, transport and lots of people involved.
And wait for it..............you can do ALL this with DOLLARS too !!!!!!! Or any fiat for that matter
Law enforcement will find a way to get the bad guys using BTC
Just hope good guys don't get caught up in the paranoia probes