Short take I don't have time to develop now but will later: basically the Hibberd book presents D&D as rattled about how bad TV-Dorne went in Season 5....but they approached it with an "it's never my fault!" mentality. The warped "lesson" they took was "no new characters!"
The warped "lesson" they took from the failure of S5 Dorne was "it's GRRM's fault for having so many characters! and introducing new ones in the middle of a long book story!"...instead of "our writing is awful, and we fetishize Dorne as 'our Brazil'..."
So there's this whole thing in the Hibberd book where they say the "lesson" they took from the failure of TV Dorne was "GRRM has too many new characters in later books - condense everything into characters we already know" (instead of building up Aegon VI and Golden Company)