It’s definitely a story that’s grown in the telling.
And I don’t think Martin can pull off an ending in two more novels. A Dance with Dragons became so unwieldy that he had to split it into two books (A Feast for Crows being the first half), and even then he had to pull some stuff out of ADWD and put it in The Winds of Winter. I don’t really see how he won’t have to add at least an eighth book, and probably a ninth one, too. (The Wolves of Autumn and The Lions of Summer between The Winds of Winter and A Dream of Spring?)
But I suspect that when you combine Martin’s age, his slowing output, and the ever expanding story, that while we may eventually hold TWOW in our hands in the next five years, it will be the last novel in the series that gets published unless Martin hands the series off to someone else to complete.
As for the tv series, it’s interesting to see it unfold. The first four seasons were a pretty straight-up adaption of the first three novels, and then they struck out on their own and started removing characters (even major ones) and entire plotlines. Weiss and Beinhoff probably saw the writing on the wall and they had focus on tying everything up and finishing the story off in seven or eight seasons.
It did lead to some continuity issues, like with the season 7 episode “Beyond the Wall,” which would seem to take place over the course of a few hours or a day, but would, realistically, probably take at least a week for all of that stuff to happen. Overall a horribly clumsy episode that has so many elements that don’t stand up to scrutiny that it’s a huge blemish on an otherwise remarkable series. I understand they had to accomplish a lot of things, but you start asking yourself questions like how did Jon not freeze to death after falling in the water? Where did the Night King get those enormous chains? Why didn’t the dragons kill all of the wights and white walkers? Wouldn’t the weight of a dragon landing on the frozen lake break the ice? Why didn’t the Night King kill the dragon right in front of him? How fast can Gendry run? That run should have killed him considering he spent his whole life in the subtropical south. He’s probably never seen snow or had to wear a coat.