In this day and age of hyper-serialized entertainment, I’m sure there’s lots of pressure to make everything a series. But indeed, there’s nothing wrong with writing a one-off novel. Imagine The Name of the Wind, but instead of the final section with that dragon (which I have mixed feelings on), we see the tale resolved? Or if A Song of Ice and Fire concluded on Book 3? Though this, I think, would require some serious rejiggering of the story. But heck, what if the Red Wedding had been the end of the series? That would be about as bleak as you could get.
A big part of Lord of the Rings’ influence is the prevalence of The Trilogy in fantasy literature. But not every writer is up to the task of a sprawling epic tale. I kinda think GRRM is an okay writer with an amazing idea, and that’s why he’s struggling with it.