For me personally I can name numerous declines which where ether compensated for or where not. The one decline which I can clearly say made the Manga rather silly for me was the revival of the entire deceased Naruto cast with the exception of one particular Senin. In dragon ball constant character revival was irritating but it was part of that story’s universes laws. In Naruto the first and second Kage’s where revived however that revival was fairly limited and it did fit into that world’s laws.
The mass revival however made death in the series lose some of its consequence. Matters which had been closed where reopened for no good reason other then filler and maybe fan service in my opinion. The Hokage revival especially after being made impossible in the third hokage - Orochimaruo fight (or so we had been told) went against rules I had come to appreciate as… well the settings rules.
Since that final act of revival I have been unable to take anything seriously in the series. Its not too bad now because I have simply stopped taking things seriously in the series and have since then resumed enjoying however it did damage the credibility of the Naruto universe and my faith in it.
I just treat everything now as a pillow fight, if one of those pillows turns out to be SO6P I won’t be surprised because there is no longer any consistency in setting and its universe’s laws which can be counted on. It is as if physics where to be revealed tomorrow to be of no consequence and actually we can all disobey gravity if the plot wants us to.
Obito was in many way’s the final straw.
I still enjoy the Manga, I do not enjoy it for what I enjoyed it for previously but as a bit of silly fun with some remaining charm it still can be enjoyable. I think an expanded Pain arc should have been the climax of the series, that was at least for me the climax. After it there where still interesting things going on but nothing has reached that “high” ever since, in my opinion.
I must say however that this is just my opinion, we all enjoy different thing. I am not saying my view is how it should have been and what is obviously wrong, I am only saying the things which made me personally have problems with the series.