Naruto ending, while it did lessen the sales of the magazine, did not really cause much harm to the magazine. One Piece is insanely popular on its own to warrant big bucks and the newer audience is enjoying the hell out of Academia and Food Wars. So saying that the magazine is doomed just because of some megahits ending means ****.
The 80's had nothing but Seiya, Dragon Ball, JoJo, and Fist of the North Star. And it still did fine when Dragon Ball Z came into the picture when other hits like Yugioh, Shaman King, Rurouni Kenshin, Yu Yu Hakusho, and Luckyman existed.