you're assuming all work finds a publisher and success in the market. There are numerous examples where the opposite occurs. For films in the US, scripts which are offered for sale are typically registered with the Writer's Guild for purposes of establishing copyright (indeed most producers will refuse to look at a script that is is not WGA-registered, in order to avoid legal disputes). But it often takes a script years to go through development and get turned into a movie - oscar-winning Dallas Buyer's Club floated around Hollywood for 20 years before production and release, and there are many other example: http://en.wikipedia.org/wiki/Development_hell#Films Should the writer of Dallas Buyer's Club only be allowed to receive royalties until 2020, insofar as they can be traced to the 1992 script?
Meanwhile, the stories of authors who published to indifference only to see their work become popular many years later are too numerous to mention. Many artists sacrifice their economic wellbeing on the altar of their artistic endeavor, and in matters of individual copyright ownership it's not unreasonable to take that into account.
The term for royalties doesn't have to be linked to the term for copyright. If all scripts already pass through the WGA, they could impose conditions on producers for royalties even after the script is out of copyright. They could use NDAs to prevent producers from stealing scripts. If there is one thing Hollywood shows us, it is that collective bargaining is a very powerful tool.
Meanwhile, the stories of authors who published to indifference only to see their work become popular many years later are too numerous to mention. Many artists sacrifice their economic wellbeing on the altar of their artistic endeavor, and in matters of individual copyright ownership it's not unreasonable to take that into account.