Again, I think it's a mistake to hold up GRRM as representative of someone who would be "shafted" if HBO were able to make GoT without his participation. That ignores the other side of the coin: all the works that
could be made into other media but aren't because: the author doesn't want someone else to sully their work; the author wants too much money; no one can find the copyright holder.
It's interesting that none of the works you cite as being vulnerable are more than 12 years old. If we could agree on 12 years with a 12 year renewal, I'd happily compromise on that compared to what we have now.
Looking at Cassandra Khaw: the book you mention, Hammers On Bone, is just seven years old. So under my scheme (I allowed for the idea of renewals) maybe she'd be on her second renewal? Certainly nothing like the current system, where that book will conservatively be under copyright well into the 2100s.
But to your point: U.S. publishers sell almost a billion books per year. I'm not sure how much authors get, since the numbers seem to be different for professionally published books vs. self-published, but let's take it as $1 per copy. HoB is currently ranked 560,000th on Amazon. If we take any kind of reasonable distribution of book sales, that would have her selling <= 1,000 books per year. That means she's clearing up to $1,000 per year on that book -- certainly enough to call it "significant revenue," but also not enough to keep body and soul together on its own.
And again, I'll point out that current copyright goes far beyond the death of the author. Should we be similarly sympathetic to the children (or even grandchildren) of the author?
And as an aside: Cassandra Khaw is actually similar to Piers Anthony: the very first book in the Xanth series is currently ranked 48,000th at Amazon, but the 20th book is ranked over 1 millionth. Interestingly, the most recent book is ranked 280,000th, so better than the middle of the series, but not as good as the first book. This speaks more directly to your point than any of the examples you gave, since A Spell for Chameleon came out almost fifty years ago. And the Battle Circle omnibus edition (it was a trilogy originally) is ranked 180,000th, so better than the most recent Xanth novel.
But all of this ignores the point I originally made, which is that copyright is only secondarily for the benefit of authors. If you really want to argue for longer copyright, you have to support the idea that fewer authors would write, or write less, if copyright were shorter. That's the only justification that speaks to the intent in the constitution.