I think part of it also depends on how much we have a signal that the author themselves has given up the farm.
For example, Harry Potter and Star Wars are clearly IPs in which the creator has zero qualms handing over the rights for all kinds of extensions of the universe, movies, tv shows, other books, plays, toys, amusement parks, etc.
This pretty easily removes these works from the sphere of art and more into the sphere of "pop culture".
In the GRRM case, we already have some extensions of the IP, tv shows, board games, video games. However, it's not nearly as severe as some others.
In either case, I still think anyone who thinks they can "complete" a series by an author doesn't fundamentally understand art. It's a logical impossibility. I'd feel just as uncomfortable with this proposition applied to JK as I do to GRRM. Authorial intent matters. Even if the author is "dead" once the work leaves their finger tips, art is not a perception of the natural world, it is a form of expression which makes the particular human being behind it essential to its constitution, for better or worse. Many people understand this intuitively. The death of the author is more about the fact that an author cannot control the interpretations that ensue once a work is published, it's not about the author somehow being totally fungible.
Even in cases in which works are "completed" by other parties (because the original author died and asked them to do it, for instance) the parties involved have a clear notion of trying their best to do it "as the author would have intended". This is impossible, but it speaks to the human understanding that the artist is in fact in some way an essential component of the art.
"Completing" a source work is way different than spinning up a derivative work, partly because the primary author is not involved, so the derivative is, definitionally, not an instance of their self-expression or understanding or framing of the world. It's a totally different thing.
As modernmech put it, a conception of art without the artist or humanities without the humans is basically just a fully commoditized conception of art. It is not art at that point. It is art reduced to goods and services.