I'm not sure I would say they are idiots. I don't really know what these guys did, but the first several seasons were pretty great. What we know now (and maybe I'm the only one that didn't know this before), is that HBO encouraged them to spend some money on real writers, and they declined. They decided they could write the final seasons themselves, with some help from their "assistant." They felt "they had to make it their own." They thought the scope of the saga was "too big" to boil down and understand, but say "the show was about power."
We know from the past 10-15 years in film that big budgets are used to make effects, epic sets, and CGI. That's what works, and makes money, but its hard to make something special without paying good writers. These guys were pretty good at the screenwriting or adaptation when they had the books, and that is a pretty rare skill, but they aren't good writers. They had good actors, great sets and effects, but didn't have good writers.