Stannis isn't an inflexible character in the books though. Far from it, he's the most progressive and willing to change leader in the books. People read Donal Noye's quote about Stannis being pure iron and assume it's true. It's not.
Benioff probably not seeing as he actually is a credentialled and successful writer, but Weiss consistently failed and never did anything. So yeah, we probably could write better than him. BTW, it's not really that one needs to write a better story. There's a perfectly good story already written by a man named George R.R Martin that they could use instead of making up their own story.