@Master:
Death Note, Bleach, and Naruto won't stay at the top forever.
Consider that the six year old Inu-Yasha is still popular as an anime and manga despite the rough consensus that it is a travesty of storytelling. Eight year old DBZ also remains a fiercely popular title despite having been off TV for a while. Pokemon, while losing its 400% steam from its heyday ten years ago, is still a massive multimedia franchise. Ranma 1/2 is twenty years old and still serves as a manga standard, if not an anime one.
I think that once a series gets on top, it'll continue to stay on top for many, many years. Demographic tastes don't age, but people do and so move out of the demographic. The Bleach fan at age 15 will be replaced by another 15 year old when he/she becomes 18.
Personally, I feel the whole dream some people have of OP becoming an international phenomenon is pretty far-fetched, and from my perspective quite undesirable.