I agree to a certain extent. It was undoubtedly difficult to try to complete the story that, as you point out, GRRM is having trouble completing. The debts they have to pay off are adding up faster then they have the ability to do so.
At the same time, they fuuuuuuucked it up by making insane leaps of logic, dropping any character development, and having characters do things that were counter to the journey they were on (see: Daenerys)
Ultimately, D&D screwed up by insisting on shorter seasons towards the end. HBO was willing to pay for full seasons but D&D refused.