as the coins spread out through the economy though it's extremely difficult to link illegal activity with those original coins.
Some research has shown that technologically it is not usually difficult to link activity on Bitcoin.
Perhaps you are making the argument that it won't be politically or operationally feasible to do so. I offer the exhibits of England's Admiralty law and the recent application and increase of Civil Asset Forfeiture in the USA (and essentially in Europe too when they seize your cash at border inspections). And the centralization of mining is the operational solution.