Try to follow along... No one is claiming that a copyright ceases to exist upon the expiration of the work's creator. Of course it exists. The question is if their are no interested heirs or assigns, is that copyright moot? Copyrights are to protect the author and its estate. If there is no author nor any estate, in other words, no one holding the copyright, who or what is the copyright protecting?
Many notables with copyrighted works fold leaving legal entities to hold and enforce those copyrights. Many do not. In the latter case, if there is no interested party then 'getting away with it' [your words, not mine] would seem to pretty much be the case. Just who would be coming after you? It is, after all, a civil matter requiring at least two parties to be in contention. Who would be the second party?
Could it be the case that if someone dies and no entity picks up the ball for copyrights held by the dead guy, ever, could those works then be considered to functionally be in the public domain? That is not to say that at some later date an qualified interested party may exist and enforce the copyrights even retroactively. But that may never be the case as well.
I have questions, not necessarily answers. I don't know, do you? Really?