I don't know that A Game of Thrones is a good example, at all.

The series was already remarkable commercial success before the TV adaptation. A Feast for Crows debuted at #1 on the NYT list in 2005.

The series sold millions of copies prior to the TV series. That's more successful than the average successful Fantasy novel by orders of magnitude.

If the books sold even more copies after being adapted, that's because HBO put the story on TV, not because of anything the author did.

And, of course, even if the first book in the series lost it's copyright after 28 years (nearly three decades!), the all the rest of books in the series would still under copyright, and the HBO wouldn't be able to access the ending without the authors help, as it hasn't even been published yet. The most HBO could have done without Martin's involvement would have been to create glorified fan fiction, while leaving themselves open to lawsuits about any similarities to any later books in the series under copyright.

Almost all the money almost any artist makes comes in the first 28 years. It is hard to see why we should deprive all of society from benefiting from using, building on, or remixing culture, to slightly increase the leverage that a handful of exceptionally rare winners get.

An of course, there is a huge gap between 14+14 and today's maximalist copyright regime.