I'm not ready to write off D&D as Star Wars creators. Who knows? When they aren't cribbing off of someone's incredible blueprint and they get to build it up all on their own, they may actually be able to make a decent film trilogy.
I don't believe that the collapse of this TV series can solely be attributed to any one or two people. I think D&D can take a fair share of the blame They didn't take their time with these last two seasons, which required way better pacing and deliberation than they received. HBO even offered them more episodes and, as I understand it, they declined.
I'm certain HBO had a role to play in this somehow as well. Maybe HBO was demanding more deviation from the source and D&D didn't want to do that. Maybe there was pressure on them not only to continue but to do so in a way they considered unsavory. For all we know, they wanted to remain faithful to Martin's intentions but the studio was asking them to do it in a way they didn't want to so they said 'fuck it, we out.'
I'm also going to lay some blame at Martin's feet, and you all know I think he's a great writer. Dude could have at least had Winds of Winter done by the time season 7 started shooting. Then again, maybe an entire different kind of backlash might have occurred had he gotten it out after seasons 6 and 7 and pissed off a completely different percentage of the fan base. Not having the entire story done before they started adapting it was a big problem no matter how you slice it and that is partly Martin's fault. Releasing two books in an ongoing series in 19 years is fucking clown shoes.
For something this surefire to be this far off the rails is usually a cluster by committee.