Such a signed message is a necessary minimum to even start the conversation, but it is not proof of identity on its own.
And it'll be even less proof overtime, especially when someone accomplishes to solve the ECDLP and have Satoshi's public key reversed. At some point in the future, even his
PGP key might become useless to have, in case that cryptography is also broken, and make it (finally) impossible to have a proof of identity for that person.
There should be nothing at a technical level to stop that from happening, but I am still of the opinion that doing so is theft.
What I'm arguing is that there should be nothing at a legal level to stop that neither, unless there a way for the original owner to provide a zero knowledge proof. You can't sue someone for picking up physical cash that isn't theirs, but you can sue them for picking up electronic cash? Regardless of the latter being less moral or not.
If I know your credit card number
Woah, I'll stop you right there. Credit card ownership isn't granted from a CVV number, but from an entire legal framework, wherein I'm legally in possession of that money according to the credit card company. The card's pin is only used to authenticate this ownership.
There are 8 billion people on the planet, with 8 billion different ethics codes. That's why I'm saying it's highly subjective
Recent even from the USA: Tornado Cash mixer's developers arrested, because... Apparently it's immoral, and therefore illegal, for some people to let some individuals protect their privacy, when a part of those uses it to launder money. Should I feel immoral for wearing this signature? Hell no. Quite the opposite.