Yeah, I've been grousing about this since I watched it.
They managed to double down by having bad strategy in the next episode. Division of forces, use of sea lanes that you know the enemy has superiority in, lack of scouting ect.
The writers are going for story beats on drama rather than sense. I doubt they're following Martin's ideas too closely. They themselves said they made the choice with Arya and the Night King when they diverged three years ago and were not beholden to the books on that. D&D said that at the core they made the decision because it was "unexpected and dramatic" not and specifically because it hadn't been seeded in the story.
In many ways they've gutted the entire point of the story that was set up in the first scene. The overlying theme was that the petty squabbles, nobles grasping for power, selfish interests were a side show to the threat from the north. That everyone needed to put aside their differences and unite for a common goal of survival and that to do otherwise was pure folly.
The entire point of the phrase "Winter is coming" was to say "Always be prepared, we'll need to work together to survive."
The show abandoned that by having the work of mankind and their unity mean almost nothing, and have the "Great War" resolved in one night. In the end Cersei was the smartest person in the show, she was right. Sending her soldiers to help would've been useless. The only play was to cast the dice and hope the north won and left themselves weakened.
I think it's telling that GRRM's books are "A Song of Ice and Fire" and this show is "Game of Thrones". In the end the Game of Thrones is all the show is really concerned with, and it's poorer for it.
