Well, truth be told, yes, Naruto sales did decrease. Imo this is because Kishi introduced too many characters and switched a lot of focus. I think that whenever the original cast, Team 7 and the ones close related to them is in focus, that's when the audience comes back again, so to speak.
I am curious how the sales are going to go from now on tbh, given that Kishi started to kinda switch the focus back on Team 7...more or less, but anyway, you get the idea.
I'll say this once because this discussion is valid as who to blame for low ppv buys. Basically, what I'm trying to say is it the manga fault? Is it time has changed fault? Is it advertisement fault? What's the answer? Simple: who knows.
For manga wise, I believe you can get the answer from weekly rankings, and yes, Naruto has been in top 5 for a whole year. I believe it was in 7 due to Hashirama's flashback chapter 3. Anyway, my point is the ranking can determine the popularity in a way, not guaranteed. As for sales, that's all for speculation and that's how it is. As for my ppv comparison, put it this way. When it gets low ppv but rate, who do you blame? Weak advertisement? Time has changed? Schedule planning? One superstar not being that top star? Who knows.
One thing that I'm surprised and to be honest, if you feel declined, ask yourself this: why it didn't suffer Bleach's fate? Beezlebub is going downhill. Bleach is struggling bad. Other manga who still didn't get animated is going down quick. You can say Naruto is just too popular, but anyone can fall.
Basically, many of us could fall down the line of "That was terrible" only for another to say "I like it". Then you'll be like "wtf". We're like CoD haters, we believe we are millions of people that previously buy the game, only to see the new game sell the same if not more.
Long story short, the world will never know.