That is a good point, but we might also end up with yet another layer of criminal organization: "dark exchanges" that allow victims of cybercrime to get cryptocurrency without interacting with the legal financial system. While I would rather that ransoms not be paid at all, if a ransom has to be paid, I'd much rather the company get their cash in a legitimate manner, interacting with financial entities that cooperate with law enforcement, rather than via fences and money launderers.
At some point, companies can just arrange for the transfer of suitcases full of cash. The point is that increasing the friction on these crimes reduces their incidence, and that's a point the author provides evidence for.
Consider there is a war on drugs which has repeatedly proven that "increasing the friction" might actually make the situation worse. Broadly criminalizing any and all exchange of money to unauthorized tokens (any more specific definition of a cryptocurrency will be subverted) is likely to end up in that territory.
