The isotopes were just an arbitrary example. The important bit was at the end "or something else we don't know yet". Granted I'd guess that's fairly unlikely for coins specifically.

> currently just disappear

They don't though. The preferences are as other comments have described - to stash the artifacts away in storage or to leave them in the ground for excavation in the future.

Whether or not the laws and common practices are fair to the people who discover the items is a separate matter.