The public domain has value for society that outweighs the value most literary works will have for heirs generations after the author's death. (Ironically, some of the older literary gems we do still remember are only in print now because they became public domain.)
How much of that is more due to clarity of rights rather than the work being in the public domain?
I've used public domain materials before. If you use works before 1923 (might have been 1922 or 1921 when I started), the rights were clear. Unless the work was first published outside the U.S., material published before 1923 was clearly in the public domain. So, the rights were easy.
For works first published in the U.S. from 1923 to 1969(?), the copyright had to be renewed every 28 years or something. So, you would have to research the work to see if it was renewed. And that was not easy. It wasn't like you could go online and look it up in an online database.
Far easier to use material first published in the United States from before 1923 (or 1922, 1921, etc.) where the work was clearly in the public domain, so there was no question of rights.
And sometimes works are forgotten not because they aren't public domain, but because the rights aren't clear. Who owns them? Did any heirs even realize there was intellectual property in the estate? Who got the rights? So, if you wanted to license a work to use for something, who would you contact about getting a license? Or, maybe, who would you contact regarding buying the copyright?
One solution might be, well, after x number of years, it becomes public domain, then you don't have to worry about rights. And that might have been a useful way of managing things back in the old days of paper records that might take years to go through and research who owns what. And it was. Researching copyright records for works first published in the U.S. from 1923 to 1969 can be a royal pain. Easier to use the stuff from prior to 1923.
But, now we have computers and huge hard drives that can store hundreds of thousands of records, millions of records even. And, you can buy racks of hard drives that can store millions more plus maintain backups. So, the more modern solution may be to keep better track of rights ownership in a robust copyright database. Any time ownership changes, perhaps the law would require a change of copyright ownership be filed to update the database. That way, the ownership and rights are clear.
Then better train the public to be as aware of intellectual property as they are of physical property. Plenty of people know to riffle through grandma's goods to look for stuff they can sell on eBay; how about making sure they know to see what intellectual property she may have created? Creators should make sure wills cover their intellectual property; in case where there was no will, perhaps public education or resources for executors ought to train them to search for intellectual property so the copyrights can be assigned to an heir, sold or otherwise handled.
If finding the copyright owner is easy, if it's easier to find a contact person to inquire about licensing, then copyrighted materials can be as useful as public domain materials.