I'm surprised D&D were hired to write a new Star Wars trilogy, I seriously think they're mediocre at best when it comes to writing a new story, the writing in the latter seasons is arguably the worst part about the show, D&D have completely moved away from the moral complexity of the characters that we got in the early seasons, to me it's clear those two can't do much without GRRM's source material backing them up.
They are master salesmen even though they are lost at writing. They somehow convinced GRRM to let them do the show. Obviously the books are a masterpiece so its easy to see how the show took off. They ran out of material, gave the viewers what they wanted and now are in position to do any major project. Any studio exec that goes to their office can see all the awards and will pay them top dollar for them to do other projects.