If we lived in a perfect world, there would be no criminals. If our perfect world had criminals, then they would be "perfect criminals", so the idea that money spent to catch dumb criminals (e.g., those using bitcoin as medium of exchange) would hold even more water than in the current world.

The fact is that big criminals have long abandoned bitcoin. They use monero and cash. The biggest criminals in this world, however, are the issuers of the US dollar.