I'll keep it short and clear.
"Your Private Key, Your Coins" So if you got the chance to get any private key of any address that has funds, you are it's owner.
No. This may be true for keeping the coins for your grandkids. But people usually want to use them to buy real stuff, like houses or cars. Saying “my keys, my coins” when someone asks where your $2 million is from? That just sounds silly.
Also, this idea that having a key means ownership has its limits. If I steal someone's key, does that make it legally mine? Definitely not. This argument doesn’t hold up in the real world; it only proves a cryptographic relationship and virtual proof of access.