There has been an increase in kidnapping for ransom in some parts of my country. Many people would have thought that these criminals would ask for cryptocurrencies as payments. But they always ask for fiat cash payments, and it is almost impossible for the police to track these payments. So, besides online fiat transactions, cash might be very difficult to track.
Very true, it is almost impossible to track cash, and it is a suitable tool for scammers and other types of criminals. Take note that many criminals do not even understand anything about BTC or its technology, so they cannot commit their crime in BTC, but they use cash, which has been a tool for scammers for a very long time now.
The French authorities are also trying to get more information from authorities in Switzerland, where last year Durov's former partner made claims of violent conduct by Durov on one of the children they had together, i don't know if this is true, but it looks like the French authorities are just looking for more info to compound their claims against Durov.