Not necessary. He can then empty dead wallets from which there will be no demand. And as far as we know, there are a lot of lost wallets, especially wallets that did not have outgoing transactions after 2012. And this will not be theft, since even the real owner will not be able to confirm his right to this wallet. And you can easily, having a private key, say that it was yours, you just recently managed to find the key.

We all know that there is no system created by people that does not have vulnerabilities. Everything breaks. And Bitcoin too. It may even already be hacked, just why talk about it, withdraw your money and live happily.
Every year billions are stolen from the Visa and Maestro systems, but they don’t talk about it so you don’t panic. Even the Apple system is not secure as advertised.

Just if I could find a vulnerability that allows me to get any private key. I would quietly and calmly rummage through dead small wallets and cash out through exchangers. Why would I leak such information and put my life in danger? Such news will reset the price and many will simply lose billions, and many will want to take revenge on me for this.