Assuming only people with private key can decrypt the message, then the privacy can be kept. However i agreed, in case the private key is lost, there is no way to delete the old msgs.
Not just that. New developments could appear that may allow us to plausibly decrypt these messages. e.g. Older prime based algos with small keys are no longer used.
Yeah the public key cryptography that bitcoin uses would need to change, but so long as you don’t reuse addresses (like we all do with memo), they won’t be able to steal your money
