So maybe the fixed term is 15 years, or 20. I don't see the benefit (to society) of having uncertain terms though.
That a book takes a number years to write is irrelevant. The copyright will be dated the year it is published, not when you thought of the idea. Guess what, if you publish a new edition? You can put a new date on that.. the fixed term copyright applies to the copy that you are reproducing, not anything else.
Anyway, GoT books started getting popular well before 10 years was up. George R R Martin got rich from the first few. If copyright on them had expired, well guess what.. he is still George R R Martin and he is still creating books which would be wildly popular and make him plenty of money he could invest for his retirement. That also means that he could have trademarked the characters and situations and retain exclusivity for the stories he created. He could licence those trademarks for huge amounts of money to film studios and get films made. George R R Martin would still be incredibly wealthy with a fixed-term copyright of 10 years.
J K Rowling would be incredibly wealthy as Harry Potter and all the sequels were incredibly popular pretty much as soon as they were written.
As you say though, most authors earn a small stipend from their work. I don't believe that they actually rely on that income through their retirement though. The vast majority of their small stipends dry up in short order. I used to work in the UK book publishing industry, 25 years ago. There were approx 30,000 publishers in the UK at that time, the vast majority of which had one or two titles to their name. That suggests to me that most of the books published were effectively self-published. The authors will have had to pay for the print run, and work another job anyway.