Really? Then they probably shouldn't have wasted everyones time by showing his backstory, some of his motivations and building him up since season 2 and his relationship to Bran and have Jon Snow freaking out about having to fight him for 2 seasons. As a plot device whats the point of him or the white walkers or the army of the dead, If not to serve as a greater nebulous threat that will wipe out humanity if the people of Westeros don;t get their shit together? It's not like they needed the Army of the Dead to unite everyone against Cersei, considering the north already fucking hates her. It's such shit writing and screams of writers not knowing what to do with the arc, or being bored with it. Seriously, Cersei is a greater threat than an ancient magical being who can raise the dead? What a load of shit.
Even if you include the off-screen discussion, the Night King didn't really have that much build up overall. There were plenty of other baddies who got more screen time and were killed off early, like Joffrey and Tywin.
In terms of the greater story arc, one can say the purpose of the Night King was to forge Jon into a man who can actually realistically claim the Iron Throne. In the absence of the White Walkers, he just would have been a Crow, maybe eventually becoming lord commander in middle age.
If it doesn't work for you, fine. But it's a big mistake IMHO to presume this wasn't GRRM's plan all along. All indications that have been given suggest that the show is ending on the same broad strokes as the books would have. I doubt he plays to have Arya land the killing blow, and I suspect Cercei is swapped out for Young Griff as the final antagonist in the books, but I'm pretty confident the Night King isn't left until the next to last chapter or anything.
Well I wouldn't have had him kill the Night's King in Episode 3 for starters.
I think it's the smart choice. As has been repeatedly noted, it follows Tolkien. In Lord of the Rings, Sauron is defeated with five chapters left to go, and there's an entire secondary arc dealing with the crowning of Aragorn and the defeat of Saruman/Wormtongue. In The Hobbit, Smaug is also defeated with five chapters left to go, with much of the remainder set aside to battles over his treasure.
Simply put, there's a lot of shit yet to figure out in universe. Will Jon come forward with his true parentage to the public? Will Dany marry him, or shiv him at the last minute as a rival to the throne? Will one or both of them die for other reasons? Will Cleganebowl happen? Who is going to kill Cersei? Will Gendry end up with Arya? Will Gendry be named the Baratheon heir? What's Sansa going to do? Will Bron actually kill Tyrion or Jamie, have a change of heart, or be killed by someone else? None of this stuff can happen with the existential threat of the Night King hanging out there, because everyone is focused on the battle of The Living vs. The Dead. The current point in the arc is the denouement, which allows for some space to begin resolving some of these issues prior to the epilogue (which presumably will be the entire final episode).