Ok, so you can't get any privacy when using Bitcoin, let's say you use Monero instead.
So you kidnap someone, demand a payment in Monero and the family pays it. So now you have X Monero. Now what? In order to use it for something, either you need to purchase it with Monero (meaning you can only buy things like drugs and other illicit things, no popular stores offers Monero as payment), or you trade Monero for Y-coin and lose your privacy again or you sell Monero on a exchange for Z, again losing your privacy.
Replace Monero with any other "privacy" coin and you have the same problem.
Cryptocurrencies with public ledgers, "privacy" focused on not are simply not a good solution for crimes and preventing people from finding you. The best tool for that job is still cash.
> you trade Monero for Y-coin and lose your privacy again
This seems to work fine, though. If Y coin is Bitcoin, all anyone can tell is you traded Monero for Bitcoin. They can't go and track where the Monero came from as they could with Bitcoin. It gives plausible deniability.
So you kidnap someone, demand a payment in Monero and the family pays it. So now you have X Monero. Now what? In order to use it for something, either you need to purchase it with Monero (meaning you can only buy things like drugs and other illicit things, no popular stores offers Monero as payment), or you trade Monero for Y-coin and lose your privacy again or you sell Monero on a exchange for Z, again losing your privacy.
Replace Monero with any other "privacy" coin and you have the same problem.
Cryptocurrencies with public ledgers, "privacy" focused on not are simply not a good solution for crimes and preventing people from finding you. The best tool for that job is still cash.