How can tainted coins becomes hard to be recognized by centralized services

[Charles-Tim]

Nothing happens because tracking cash is much more difficult than following the blockchain trail.

Yes, exactly, that is why bandits, kidnappers and many other criminals prefer Fiat in cash, it is not traceable and tainted-resistant because it can not be tainted. Like the recent hack, all the addresses have been known, will be tracked and monitored, what Fiat in cash can not give. We can even see thieves robbing banks successfully without being able to trace the money stolen.