Bitcoin should not be treated as a physical asset. With a physical asset, if someone else possesses it then you do not. If you lose cash, a credit card, your wallet, your phone, or any other physical asset, then it is obvious to you that that thing has been lost as it is no longer in your possession. Losing a private key is not the same, since it is entirely possible for two people to both possess the same private key at the same time.
We agree on all points in this particular case, however the thing with this particular private key found on the beach by the OP is that
it is a physical object represented in a paper wallet form. Whoever the original owner is or who their successors are, they have lost a physical object that used to be in their possession somewhere and somehow. Hopefully, that's not their only backup, they will discover sooner or later that it's missing and move the coins elsewhere.