To be fair to D&D, I do think the ending they came up with is plausible and fitting for the GoT characters in the book.
Unfortunately, GoT did not develop the characters to get to that state of mind, nevermind the immersion breaking when GoT previous was focused on the minutae which is what made it stand out from other fantasy at the time.
Basically this.
D&D took outlines from George....and basically threw the
outlines on to the screen and called it done. What they failed to acknowledge is that an outline is just the skeleton of a story, and that it lacks all of the necessary connective tissue to really flesh out the whole tale and create characters whose actions are believable in context.
As a result, you had characters just kind of doing things because "Now is when we do those things," and events happening because "This is when the events happen."
Martin's books have also already been heavily departed from and altered in the name of "streamlining" them. Initially, I thought this was a good idea, since his books felt ponderous and meandering, but I've come to believe -- hope, really -- that Martin's meandering is actually meant to help illustrate the journey of a character so that when they hit that end point, their actions make more sense in context.
For example, when Brienne finally does XYZ in an as-yet-unreleased novel, she will do so because of the events she experienced in Book 4 wandering around the eastern coast looking for Sansa and Arya. Dany's heel turn will likely make a LOT more sense because we'll have seen her frustration and lack of understanding in why populations don't accept her benevolent rule and how they seem more content with fighting their own petty squabbles amongst themselves instead of "breaking the wheel" (which, I should note, was not in any of the books yet released).
There's also a bunch of prophecy related stuff that the show just...dropped which I expect will show up, and the Others are depicted quite differently in the books.
All of which is to say that I think D&D are...good...at running shows, but aren't so great at telling stories. They need much, much better writers around them. The first few seasons of GoT are
AMAZING and a testament to their abilities
as showrunners, but the last four or five seasons really lay bare the fact that they are not great at creating original material.