For me it wasn’t the characters surviving but how they survived.
The Dothraki we’re eliminated in seconds. Compared to the other living soldiers they were at a tactical advantage being on horseback. So how does everyone survive on foot vs the army of the dead for an hour long battle? They were easily overwhelmed but Theon with a slow loading bow and arrow wasn’t?
How does Sam being stuck on his back for 30 mins like a turtle survive? Now if Ghost had protected him I would have been fine. But that wasn’t the case. Seems like a simple fix.
How the heck did Davos make it?
It would have made sense for at least one of the minor characters and certainly more extras to die in the crypts.
How do you not have a white walker sword fight? Imagine having two white walkers battling Brianne, Podrick, and Jaime? With 2 of those three surviving while killing the 2 white walkers?
How is it that the wights sometimes act like a overwhelming swarm from world war z, but at other times like walking dead zombies within the castle?
Which brings up another question, are the dead simply resurrected humans who should act like humans? Or do they have super powers? The show uses one when convenient but also the other when convenient.
How can wights break through their stone crypt but the one brought to Kings Lansing couldn't get out of a paper bag, well box?
I would have tweaked the ending. First the undead are animated because of the night king. So why not have them simply become inanimate and drop vs disintegration?
Second I would have had Theon get up and say what is dead may never die then stumble towards the night king. The white walkers turn toward him and finish him off. This is what allows Arya to get by them.
Lastly after being stabbed and showing all the undead becoming inanimate, I would cut back to the Night King and he is in human form ala a werewolf movie. As he is dying he reveals his true identity, most likely Stark or Targaryen.