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.
I also have backups and I think they are in a safe place and I confess I have not checked if they are in place for a very long time. I just rule out the possibility that they could disappear from there, because it's not easy to get to them and therefore I hope that they are stored there safely.
Yeah, that's a good point. I also have a number of back ups stored off site, as everyone should. These backs ups are obviously highly secured and incredibly unlikely to end up on a beach somewhere (not to mention that no single back up is enough to compromise any of my wallets), but at the same time, I obviously don't check on these back ups every day. It is possible, however unlikely, that I would lose one of my back ups and not realize for a period of weeks. There is no telling how diligent or otherwise the true owner of these coins might be, and it could very well be months or even years before they realize that their paper wallet is lost.
By the way, it's a very good idea to divide the backup in such a way, that no one can use it if somehow it is found. I will definitely fix this point soon.