I think you are spot on about ASOIAF, but I don't think this is the reason GoT ended badly. Books and TV Show started diverting much earlier, and the show had substantially fewer storylines, which is a good thing in this context I think. I also suspect that the ending we got is based on notes GRRM left. Daenerys turning evil is heavily foreshadowed, Bran ending up on the throne kinda makes sense, and so does Jamie not cashing in his redemption after all. WW not being the final arc mirrors LotR, and GRRM has repeatedly stated that he loved that way of storytelling. The first book is called Game of Thrones, the ending is about that as well.
The problem in my opinion was just terrible execution: It was all really rushed and with terrible dialogue, so none of the story elements were developed in detail, and felt artificial and wooden.
On Twitter late last year, I saw what was probably the single best idea that would have greatly improved the last few episodes, and could probably have been accomplished with a single scene (and maybe still could for a future "definitive edition").
The idea is that at some point prior to the last two episodes, Danerys receives word that Mereen has fallen, and that the slavers have retaken it. Her greatest achievement in "breaking the wheel" is now gone, and her designs to be the next Aegon are no longer nearly as assured.
Building on this idea, the scene could play out with her completely losing it and threatening to burn Mereen to the ground, her advisors only barely talking her out of flying across the Narrow Sea right then and there.
Mereen's destruction is averted, but the seed has been planted in the viewer's mind: Danerys has much less to lose now, and she's willing to rain fire on a populace with enough provocation. Now when the other shoe drops and she does this on King's Landing, the turn has enough narrative foundation to feel plausible and earned.
You say that the problem is the execution. I think it goes deeper. It's impossible to execute. GRRM knew this about many plot points and that's why he changed his mind about many things by the time he had to expand on those specific plot points.
> Bran ending up on the throne kinda makes sense
If he ever gets to the end, I'm certain he'll see that this doesn't make any sense.
Or at least he won't write the same bullshit the show gave us: "he deserves the throne because he has the best story". I still want to throw the TV at the wall when I think about that.
They also did a bunch of other annoying/bad things that deviated from the early seasons (bringing along a bunch of nameless characters on adventures to just to kill them, wardrobe and makeup went in a frankly absurd direction for no obvious reason, emphasis shifted to "wow" moments in episodes instead of paced out plot, just dropped a bunch of story lines that were interesting, etc).
To put this in context: All 9 books of The Expanse were written during this period along with a six season TV show based on the same material. The two writers were involved in both actively.
I've 100% given up on ever seeing how ASOIAF ends. Same with Rothfuss's Kingkiller, it's been 11 years and I stopped caring during the previous decade
David Gerrold is never going to finish the "War Against the Chtorr" series. The last book was 1993. We're about to hit 4 decades since the last book.
GRRM wrote himself into a corner he hasn't been able to disentangle.
And the showrunners didn't have the luxury of waiting 11 years between seasons.
I do think they could still have done a better job, but it's hard to say from the outside because we don't know the budget, time and story constraints they were working under.
By the time the final few seasons came around, almost all the actors had become famous on their own right, who were taking on a ton of other shows (and I doubt they were under contract for the entire run, especially since it was never expected to last as long as it did) and it must have been a massive challenge bringing them together to film the final few seasons.
A much simpler explanation is that GOT show creators/owners paid him off not to publish the newer books, until they consider the market life of the TV show exhausted.
Unless GRRM is struggling with mental health issues, it doesn't take 11 years to finish a book. Heck, it's his series anyway. If he has trouble wrapping up all his plotlines, he can always write an 8th book to extend the story.
Even if GRRM wrote at the fastest pace he had ever written, the books would still be way behind the show.
Here are the GoT Season release years by season: 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2019.
Basically, 1 season a year, with the final season taking 2 years.
Here are the ASOIAF book release dates: 1996, 1998, 2000, 2005, 2011
So it was 2 years at its best (the first 3 books), and then it increased to 5, and then 6. He had already been slowing up badly as he tied himself up in knots, that he has been trying, unsuccessfully, to untangle for some time.
It's been 11 years since the last ASOIAF novel. I think GRRM may have written himself into a corner and is unsure how to tie up all the threads and satisfactorily end the series.