So, why hasn’t Wizards done it, then? According to some reports, Pathfinder sales have actually surpassed, or at least equaled, the sales of D&D. If it was within Wizards’ power to shut down one of their primary competitors, why would they voluntarily choose not to?
A reasonable person would expect such a gambit to fail. Lawyers have a different concept of “reasonable”. They would look for loopholes and such.
An example of a work that was taken out of the (almost) public domain is It’s a Wonderful Life. It’s main copyright was forfeited to to failure to renew. (It’s the same reason some episodes of The Andy Griffith Show are in the public domain. This no longer happens since renewal requirements were removed in US law.) So everybody and his uncle aired it during the holidays.
(It’s a quite bizarre story. See the Wikipedia page. The story rights are also involved.)
But someone noticed that the music used in the movie was still under copyright so slam bang, the whole movie magically is under copyright again.
I don’t know how RKO (then, Paramount has it now) “notified” everyone that it was their property again. But it was a famous enough case that if you were in the broadcasting business, you would have heard about it.
The general way any copyright violation is handled is that the owner’s reps keep an eye out and when they find something they start writing letters.
For something like a book, there might be options involving reproduction in other forms (e.g., other languages, ebooks and audio books) that could be claimed to still be covered. And then maybe then can claim the original book is a form of those works. Also someone might claim that the exact format of the book was the only right given up, so if you tried publishing with a different font, different layout, etc. that is a “derivative work” not covered by the original release of rights. And on and on.
I’m sure a well paid lawyer could figure out a bunch of almost logical ways of doing this.
Messy, ugly, costly stuff.
In the media business there tends to be a lot of care to avoid pushing things in a quid pro quo way. E.g., the copyright claim on Happy Birthday is really dubious, but no one ever really challenged it because that might cause a retaliation and it would just escalate.
So if the heirs made a reasonable sounding claim of re-instating rights, with visible legal representation, most companies would back off.