How could it happen, law agencies should located the bitcoin mixer. And it's a shame for them to accept (consider as) stolen coins.
I can't believe how could they want to cash out this big amount of money, and who wants to deal with law authorities by accepting those coins? Even if you're holding it for 100 years, law enforcement agencies will always watch over it and hunt you down.
I believed that this hackers are using mixing services so that the coins are hard to tracked. Cash is still the king and the hackers probably are salivating on the bitcoins' that's why he has to moved it or probably cash it by now. I think if the authorities should exert enough effort, they can still traced it though, maybe the hackers did not used TOR to hide their identify and if you are lucky maybe the mixing services the hackers used has some data logs than can lead to them. And be able to arrest the hackers..... Well, hopefully.