It's not helping there either. Countries where the economy is strictly controlled by the government, like North Korea, are mainly using crypto and defi to centralize even more. Their government can use defi to steal from other countries with impunity, evading international laws while continuing to oppress their own citizens and disallowing them from using the internet.

>And how do those prevent rich person A from paying rich person B (by buying a painting owned by B for an inflated price) for some illegal service?

If it were done through a bank (or a law-abiding crypto exchange) they would be required to keep a log of the transaction and the legal identities of the participants, as well as a log of where the money came from and where it's going. The idea is, if buying the painting is just one of the steps to "clean" the money, they'll be able to trace it back to when the money was dirty.