I am sure that when writers start a show with a multi-season arc, they have a mental map of what they want to tell in each season. Then the studio comes along and cancels the show after they film a cliffhanger or tells them to stretch the final part of the story into 3 more seasons (and then cancels it after the first of the filler seasons flops).

From what I understand, that was not the case for Game of Thrones. Going into the last season, the show still had a great audience and ratings and HBO was eager to keep milking it. It was the writers who insisted on ending the show and had enough clout to get HBO to agree. My assumption is that they ran out of ideas to keep the show going, realized that George R. R. Martin was hoping they would come up with an ending he could use in the books, and resigned on a high trying to get other jobs before the final season ruined their reputations.