Intentional mixers aren't even the half of it. You have large exchange operators that use a single wallet. They file KYC paperwork with governments, but that's not in the blockchain. From the perspective of the blockchain their whole exchange is one big mixer. A billion dollars goes in, a hundred was tainted, a billion dollars comes back out. The only information to trace which $100 that went in is the $100 that came back out isn't in the blockchain, it's in the exchange's private accounting database.
But if you propose to taint every coin that has ever passed through a major exchange, that's pretty much all of them.
