Author George R. R. Martin began to worry about the HBO show after Game of Thrones season 5, when it began to move away from his template. Martin's popular series of fantasy novels A Song of Ice and Fire, the first of which was published in 1996, gained an even bigger boost when HBO adapted it into Game of Thrones, a series that premiered in 2011. The franchise is expanding even more in the upcoming series House of the Dragon, which is a prequel that will follow the civil war among the Targaryen family.
Game of Thrones was an enormous hit that launched many of its performers to stardom. This includes Emilia Clarke, who has since also appeared in entries in the Terminator and Star Wars franchises, as well as Richard Madden and Kit Harington, who both joined the Marvel Cinematic Universe in this month's Phase 4 entry Eternals. However, despite the show's hit status, season 8, which concluded the show in 2019, was widely regarded as a huge disappointment. The Game of Thrones finale made a lot of story choices that fans objected to, complaining that the showrunners had lost track of what made the series popular in the first place.
Winter is Coming shared an excerpt from the upcoming book Tinderbox: HBO’s Ruthless Pursuit of New Frontiers, which naturally includes a section on Game of Thrones. This section includes a quote from George R. R. Martin's representative Paul Haas, who details how the author started to "worry about the path they were [going down]" as early as season 5. The show began to go "off George’s roadmap," and he was frustrated they weren't "following [his] template." Read the full quote below:
"George loves Dan and Dave, but after season 5 he did start to worry about the path they were [going down] because George knows where the story goes. He started saying, ‘You’re not following my template’. The first 5 seasons stuck to George’s roadmap. Then they went off George’s roadmap."
Game of Thrones season 5 is certainly the place where the show started to diverge most from the source novels. This included truncating the arc of Emilia Clarke's character Daenerys Targaryen, who was supposed to have visions while traveling across the Dothraki Sea. They also moved up Tyrion meeting Daenerys to a much earlier point in the narrative, completely changing the Dornish subplot, and cutting out the introduction of Aegon Targaryen.
In fairness to Game of Thrones, George R. R. Martin has not yet published the concluding novels of A Song of Ice and Fire. Without that roadmap, the showrunners needed to create their own endpoint and begin to drive the series toward it. Unfortunately, they miscalculated what would appeal to audiences the most, but it makes sense why they would start to carve their own path to create the necessary plot threads.
Source: Winter is Coming