Hey, it's a series about a grand adventure. There are still tons of things left unsaid in One Piece. I am expecting another 10 years run.
The reason One Piece ran this long is due to its popularity in Japan. Oda said himself that he expected the series to end in 3 years. However its enormous success and popular demand forced Oda to continue the series. Now it is currently Japan's most popular manga series. As long as the quality of the series is at the present or perhaps superior level, I don't mind it to go on for another 10 or 20 years. This series is unlike Inuyasha and Shaman King's dragged on style at all. This series is THE best out there.