Reason why Naruto has lost it's quality with last few chapters is overusage of Black Zetsu. It doesn't matter that it was/was not implied that he had bigger role in this story, problem lies elsewhere. Every time Kishimoto needs to kickstart a story again, he uses Black Zetsu (Or even White Zetsul).
Kishimoto needed to revive 4 kage- use Zetsu stored inside Sasuke; Kishimoto turned Obito good and needed to force him for a few chapters to be bad- use Black Zetsu to control him; Kishimoto needed a few more chapters so he can milk the cow- Let Black Zetsu steal Minato's Kyuubi; Obito overpowered Black Zetsu's will so he can make some more good, let few chapters pass and use Black Zetsu again; Kishimoto needed to make biggest and lamest plot twists of them all- Use Black Zetsu to turn Madara into Kaguya; Kishimoto needed to cover plot holes he created- Give Black Zetsu bigger role. There are even more examples, but I think I made my point.
Worst of all, we never thought Black Zetsu had bigger role (At least I didn't), and also, I found him extremely annoying whenever he showed up into scene. Always there to make some plot twist to keep the wheel spinning. This was completely unnecessary, to have appearance of Kaguya. Madara was there before, and he was there during almost whole last arc. And even then, he had wast potential. Kishimoto could have used him in so many ways, and yet he killed him off in like 4 pages. Obito was kicking around more chapters than Madara pages. I mean WHAT THE FUCK??? Had he used him to have final fight, Naruto would have had great ending. I don't care if he had used cliches. Power of friends- sure, why not; Talk-no-jutsu- Hell, even that would have been better than Kaguya appearance. I just don't understand what did Kishimoto gain by inserting Kaguya into this story...