Long post coming. Spoilers BOTH books and show. I think D&D expected to have a lot more material to work with when they signed on to create the series. After the way seasons 5 and 6 went I think that played into their decision to shorten seasons 7 and 8. Quick timeline of events: D&D first got the idea to adapt ASOIAF in 2006. Development started in 2007. The pilot was ordered in late 2008 and shot in 2009. There were extensive reshoots and the series was picked up in 2010, finally premiering in 2011. So D&D got started right after A Feast for Crows was released. That book took 5 years from the release of A Storm of Swords, but before that GRRM was releasing a new installment every 2 years. A Dance with Dragons was finally finished in time for the first season in 2011. I think its reasonable that they believed theyd have at least 1 more book to adapt in the series before they had to start writing original content. And when they did get to that point, GRRM would have actual manuscripts and drafts from A Dream of Spring, rather than the broad strokes and key points they were given. We all know now that didnt happen. Adapting a novel is much different from writing original material, and I feel bad because D&D planned on doing the former. They signed on to tell GRRMs story, not write it for him. It was easy last season; all they had to do was take the storylines from where they left off in the books and bring them to logical conclusions. Jon was dead, he came back to life and had his confrontation with Ramsay. The Knights of the Vale help Sansa take Winterfell. Bran completes his training and heads home. Cersei has her trial (but TWIST she kills everyone). Arya finishes her training, escapes the House of Black and White and goes to work on her list. Daenerys gets the Dothraki back, kills the masters once and for all and (FINALLY) heads to Westeros. This is a big reason so much was cut from the show and altered in season 5. D&D simply couldnt introduce any elements that they couldnt wrap up in a way that impacts the main because they didnt have enough material. Now theyre in a pickle. George may have given them the ending but they now have to write the story to get us there and make it entertaining and believable for a fanbase who has been waiting 20+ years for these scenes. To make that even possible, theyve already had to cut out 7 episodes.