It's not impossible, but it can be extremely difficult.
Consider a short story published in a literary journal last week. Allow a copyright system to exist that covers each eligible work for 30 years by default, and allows multiple extensions of 5 years at a cost of $1000 per extension per work, up to a total copyright term of 100 years.
Someone in 2047 discovers a copy of this particular journal issue, which has now become obscure. They identify that this short story, though never being given acclaim, has had a subtle yet profound cultural influence. They decide to republish the story in their own commercial magazine, but want to check if the copyright has been extended. If it has not, the story is in the public domain. If is has been extended, the story is still covered by copyright.
They can search the central registry for the title of the short story and the name of the author. But they also have to search for the names of the editors of the journal, because it isn't clear whether or not the copyright stayed with the author or was transferred to the journal as part of the submission process. In fact, the copyright might have been granted to the owner of the journal, which may be an individual or may be an institution. The entry for the copyright might be under the title of the short story, or under the title for the journal, or under the title of that particular issue of the journal.
Furthermore, the author of the story, if they retained the copyright, may have submitted the story under a pen name, and/or may have registered the copyright of the short story under a different title than the title they used to submit to the journal, or the title might have been changed by the editors.
Or, the author, after having the short story accepted by the journal, may have sold it to a publisher who published it in a short fiction anthology, the copyright for which is registered completely separately and is essentially invisible to our hypothetical future researcher.
Any of these agents might have extended the copyright. It would be extremely difficult to prove to a high degree of certainty that the story was or was not covered by copyright.