Its completely trackable is most cases and should be. Exchanges should be involved in investigations, tracing addresses and fiat exit points. this is unquestionable. If they don't , bitcoin will be stuck. If the community can show some policing at least in major thefts, it would be a huge boon for the success of bitcoin. The exchanges should openly participate in theft forensics. You cna track forward in the chain from thefts and backwards from exchanges where the biggest fiat transactions should be traced backwards and see if the trails connect for suspect accounts/addresses.