IIRC, it wasn't the Beane counters - HBO actually want more GOT, it was D&D who wanted out and decided that they would take the 13 episodes in seasons 7 & 8 and shoe horn it into a scripted ending.
I read somewhere on the inter-webs, (this is extremely condensed) that GRR wrote the story to grow organically, he would create a gardening bed and place the characters in a situation and ask, how would they react and that is what he wrote. It was very organic, that is what in essence created the depth and all of the various plots and character arcs and the story would grow with many branches and twists and turns; it also became a B word to manage (apparently the other way to write a story is to have a very rigid framework that you follow all the way through). Once the decision was made to end the series D&D knew the ending they wanted and gave themselves 13 episodes to move the characters along to be able to fit the ending, which in essence is the rigid framework method. That is what has made the last couple seasons seem less cohesive overall and gets us to where we are today.