The popularity was waning since the beginning of the war arc. Its not only the ending, Naruto's been suffering with this for quite a while
If it is not after the pain arc, the general consensus should be the war arc. It was horrible, and I will always deem it as the worst arc of the entire series. So many flashbacks and villains.
I hope Bleach is not doing the same thing as Naruto with its war arc going on right now.