the criminals will have an inside man.

it is called a fence

But the problem with this is it shows a direct connection since you can follow the money. Like lets put it this way. Unless if the fence is going to pay with food, housing, etc. At some point the criminal will need to buy something or use normal money. Which means there is a connection on the ledger.

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criminals will continue to use fiat

Note is the law (USA) 31 U.S.C. Sec. 5103 https://www.law.cornell.edu/uscode/text/31/5103

United States coins and currency (including Federal reserve notes and circulating notes of Federal reserve banks and national banks) are legal tender for all debts, public charges, taxes, and dues. Foreign gold or silver coins are not legal tender for debts.

This makes it and forces it where the loan system (more so legal loan system) has to use whatever the gov marks as legal tender to pay for debts. Meaning if the company you buy your gas, food, etc from wants to stay in business. They must accept legal tender.

All that would be required for a CBDC to be forced in a way I'm talking about is it working, and this law to be slightly changed. They could even mark "Federal reserve notes" as not legal tender which automatically forces banks and other places to not accept it for debts.

Note this doesn't stop private companies from accepting whatever. But if the money machines stop making more, the gov starts recalling and burning all cash (which is actually a normal process), and what is out there just goes away over time. At a given point only collectors will have physical cash.

I can go deeper, but I hope you get my point by now