In some parts of the EU, it’s still pretty common to do the down payment of a private real estate transaction in cash, and that can easily exceed €10K. I’ve done it myself and I’m not even a local.

If it’s illegal then it’s just going to go off the books, at a small cost to taxes and accuracy of records, for some people; but for others it will force them to end this traditional practice, and I can imagine that might cause hardship.

But yeah, Russians under the bed, sure.