Wow, I had no idea so much of fandom was angry with this until I came here.
I for one loved last night's episode! Not all of it--particularly the dumb writing where they send the entire Dothraki into an unwinnable charge and act shocked they're totally obliterated--but in general it was top-notch entertainment. And, ahem, unlike some endgames, it featured breathless suspense and kept me on the edge of my seat.
Here's the thing with the White Walkers: We always knew they were going to win. The brilliance of this episode is that for most people (pretty much everyone I haven't met in this thread) were tricked into being on the edge of their seat into thinking it was hopeless, the heroes were doomed. It was game over and everyone was going to die. Of course that was never going to be the actual ending, so by default we know that somehow the heroes will win, even if subconsciously. The success of the episode is that it is so intense by focusing on the quiet, the dark, the gloom of death, that many viewers at least seems to give in to the despair it invites. And then a character no one expected to kill the Night King doing it coming off as some kind of miracle.
Now I think you could have built up to this episode more. You could've done a whole episode about the fall of the Wall, even have Jon and Daenerys there and have them see the wrath of Viserion with their own eyes as the Wall tumbles, perhaps realizing they cannot fight the Night King head to head on dragons because he brings a debilitating storm. Besides cost restrictions though, I also see why they just made this one episode and kept the rest of the focus on the characters.
At the end of the day, Game of Thrones is a human drama. That is its appeal as a fantasy series, so it must always end as a human conflict between the living. If the White Walkers are a force of nature, there is always a new world to be built after the hurricane or earthquake, or devastation of your choice. That still leaves Daenerys, Cersei, and Jon Snow. And unlike victory over the Night King, there isn't a clear obvious resolution to the coming conflict. It could be any of them (well, not Cersei), it also could be none of them. Political machinations could tear alliances asunder that were inconsequential while fighting the Night King.
I'm honestly shocked so many thought the White Walkers were making it to the finale. I mean, you could've flipped it and had them arrived in the fifth episode, but then whoever was left at that point obviously would win because... spoiler... the White Walkers always were going to lose. I think other than the Dothraki nonsense they made an effective finale to this conflict.
As for Arya "stealing" Jon's moment... eh, that was the best aspect of it. Prophecies have always been wrong on Game of Thrones, and I never felt like Jon had to be the one to kill the Night King. If anything that might've been actually anti-climactic because that is exactly what everyone expected. Arya was unexpected but not undeserved. She's been training her whole life to take matters into her own hands and not need to rely on someone else. She also learned stealth in a way Jon and the Westerosi never did. Personally, I think she was lying in wait for the Night King to isolate himself and then pounced. It was thrilling stuff.
Again, I could see them building up to this moment with another battle that the living lose, but it always had to end with someone killing the Night King, I'm a little surprised there is so much soreness over the fact it was Arya and it came after a battle where the Dead decimated the living.