As I understand PKI, and my understanding is limited so I could of course be very wrong, but in the scenario you describe the deletion of the private key would

As I understand PKI, and my understanding is limited so I could of course be very wrong, but in the scenario you describe the deletion of the private key would do nothing to prevent people from decrypting data readable with a public key. This would work with a symmetric key! But as you say, I'm struggling to think of any real advantages to using a blockchain here.