D and D got a lot of heat for the last season of Game of Thrones, but I've never thought they were entirely, or even chiefly, to blame. Most of the problem really is that GRRM obviously desperately needed an editor to rein him in as the series went along, but for whatever reason, that didn't happen. So now he has this huge, sprawling mess of a story that's going in eighteen different directions at once, and just as D and D couldn't manage to tie it all together, neither can he.
IIRC from some article or interview way back then, Martin had provided D&D an outline of all the major plot points he intended through the end of the series. So while they might not have had the specifics, the major points would have been there.
If that was true, then it would make sense that they used those major points for the basis of the rest of the show. After the abysmal reception of those points by the fans, I would imagine Martin would have stopped to think about his plans, possibly losing interest entirely.
That assumes that he did in fact provide those major points to D&D in the first place to have adapted however.
I always felt that one of the main problems with GoT/ASOIAF was that it was a nuanced, political fantasy with top class world-building, but the overarching plot was pushing everyone towards a massive final confrontation (or 2 really). There was not really a good way to resolve the confrontation without a massive battle (or 2). So the ending was always going to have to move away from what made the series interesting/successful (book and TV), i.e. plot, characters, intrigue, shades of grey.
There were other problems as well, but that was something baked into the whole series by GRRM, and I'm not sure he can really find a way to do it differently. He might come up with a different outcome of the final confrontations, but it still has to be done with epic battles.
The Hobbit ends in a massive battle that seemed made for TV, but Bilbo gets a bonk on the head. I'm sure people don't need to hear about the ebb and flow of the battle. Have King Stannis host a feast afterwards and the few characters left alive can trade highlight stories.
So the ending was always going to have to move away from what made the series interesting/successful
While you have a point, I don't think the ending was necessarily bad because of that.
To me the resolutions they did use were just badly executed. I'd have been fine with the battle of the bastards resolving the Bolton plot, if the battle as shown didn't make me scream at my screen every 2 minutes from all the logic holes. Same with the fall of Highgarden, Daenerys going insane, Bran becoming King etc. They could have reached mostly the same outcomes and it could have been fine. But the build up and the attention to detail just weren't there at all. And it wasn't even that they ran out of time, they deliberately shortened the last 2 seasons because they wanted it to be over.
I agree the execution of the end of GoT was bad (i.e. the problems weren't just the issue of everything needing to come to a head). There were a lot of different complaints about how GoT ended, but I definitely saw a lot about how it was all just battles in the last season and no nuance. I think that was always going to be hard to avoid given how GRRM had set up the main plot. And I think he will find it hard to avoid when writing the last 2 books, which could be part of the reason he doesn't want to do it.
Do you know what's nuts? He's well respected within the industry as an editor - he's worked with many of the luminaries of science fiction and fantasy over the years as the editor for Wild Cards and other short story collections.