It's absolutely the producer's fault. They actively choose to release the product they did instead of making more episodes, taking long, bringing other people in to help, etc.

Martin has claimed he flew to HBO to convince them to do 10 seasons of 10 episodes instead of the 8 seasons with just 8 episodes in the final one [1]. It was straight up just D.B. Weiss and David Benioff call how the series ended.

[1]: https://variety.com/2022/tv/news/george-rr-martin-shut-out-g...