The show runners did screw up and so did those above them, but the problems all start with Martin. He has yet to finish the book and he gave them a terrible ending to his story to try to pull off. On top of that he had so many plot threads that went nowhere, and a writer can do that with some and get away with it but not in the quantity he has done. Honestly, he set a up a great world, but the storylines and their execution are textbook examples of bad writing.
I agree with you about the show runners getting bored, but someone above them should have identified that the show was running into serious problems and solved it. Heck, the lighting was a disaster and no one at HBO seemed to notice or to say anything. Another red flag is when the winner of the Iron Throne is a character that was so useless he was ignored for an entire season. HBO should have known that such an ending wasn't going to work and Martin should have as well. So HBO is hoping that fans are still willing to watch the spinoffs after the disaster of the last few seasons of Game of Thrones. I don't know if they will, but I do know that fans do move on to other series when one wraps.
