Inadvertently, you are though. The first few seasons had the benefit of having s fantastic storyline already built, all they had to so was cut out some of the not as important stuff because on a screen, you dont get the luxury of time. The problem was always going to be how long do you tell this story on screen? You could make 15 seasons out of the books that are already written. Nevermind that the books were always meant to be slow played, with an 'infinite'timeline. Again, the TV show doesnt have the luxury of that.
The last couple seasons didnt benefit from a pre designed storyline. They had to make it up as they went and I dont think the disagreement is that writing coulsnt be better. There are definitely problems in seasons and 7 and 8. The disagreement comes, I think, in the sense of what you want to play out. Adding more NK and Bran backstory goes down a road that quite honestly, isnt all that interesting to the common viewer. More wouldve been nice, but going too in depth loses the audience. Spelling out every little detail is costly and tiresome, and you have to keep character timelines in mind, as well. The show does t get the benefit of keeping Maisie William's as a small girl forever, but the show timeline is much shorter than how long it took to film, etc.
Theres a lot more to consider than just " they had more time, they just didn't want to use it."
I'd have loved 10 seasons, but even then, is that enough? GRRM did so well in going through details that I think if you tried to portray that all, you lose half your audience due to "non action", and I think the general reaction to episode 1 and 2 of this season even speaks to that.