Naruto could outsell Slam Dunk if it continues selling at it's current pace. Maybe two more years worth of volumes to join the 100,000,000 club.
At the rate it's going, Naruto will only last for another 5-10 volumes. It has about 40 million volumes between it and Slam Dunk. I don't think that it will make it before the series ends. It just doesn't seem likely. It will probably sell over 100 million sooner or later, MAYBE before the series ends, but that would take A LOT of sales of old volumes, because it has about 20 million volumes to go, and then ANOTHER 20 million to catch up to where Slam Dunk is now.
But it's already gotten into the top 10 . It's in 5th place with nearly 90 million sales. Even when the manga ends, there are people who still will be buying the manga if they are beginners or whatever. I'm just saying as the time goes but would it reach 1 or 2?
Impossible. Naruto would have to be twice as long as it is now to outsell Dragon Ball, One Piece, and Kochikame. It just isn't as popular (or in Kochikame's case, as long) as the others. It's still QUITE popular, but not exactly "eternal pop culture icon" popular, which Dragon Ball, One Piece, and Kochikame are. And long, they're also long. Kochikame is already the longest manga in history, and One Piece will probably be about 100 volumes long itself, hopefully. Naruto will be about 50, do the math.
Also have to think about worldwide sales as well, not just Japan, as we know manga is getting more popular throughout UK, America and other countries. Man it would be nice to see Kishimoto's series be #1. ^___^
Oh, we're talking Japan here. Foreign sales are a completely different story. Speaking for America, Naruto is the most popular manga here. By a large margin. You see, Bleach is the second most popular manga, and it's roughly HALF as popular as Naruto. One Piece is 27th most popular (%$^& you, 4Kids), and it's about 14% as popular as Naruto. But ultimately, it's Japan's artform, so its THEIR accounting that matters. At least, that's what the One Piece fandom consoles itself with.