I think it's simpler than this. He started writing the first book in 1991, the beginning of an optimistic decade when a grim dark fantasy series would be contrarian and hit hard. In 2020, the real world has become much darker than it was in 1991, and nobody wants to read or write a grim fantasy story. He no longer feels badass for writing a dark fantasy during an optimistic time, so he doesn't write it! Another manifestation of this phenomenon is Charlie Brooker stopping work on Black Mirror. Brooker explicitly said that the world has become so dystopian that he no longer feels motivated to write dystopian fiction.
I think it's simpler than this. He started writing the first book in 1991, the beginning of an optimistic decade when a grim dark fantasy series would be contrarian and hit hard. In 2020, the real world has become much darker than it was in 1991, and nobody wants to read or write a grim fantasy story. He no longer feels badass for writing a dark fantasy during an optimistic time, so he doesn't write it! Another manifestation of this phenomenon is Charlie Brooker stopping work on Black Mirror. Brooker explicitly said that the world has become so dystopian that he no longer feels motivated to write dystopian fiction.