I recently read an article on Sankaku describing this situation. Not going to post a link because that site is very NSFW.
Top mangaka Ken Akamatsu has stated his misgivings about the reliance of the manga industry on the shonen holy trinity of One Piece, Naruto and Bleach, and what would befall publishers if these series were to conclude.
After describing some of the worsening financial results posted by top manga publishers, he describes his uneasiness about the situation:
[QUOTE]It’s all pretty dire, although manga won’t drop dead just yet.
However, they are relying on the bubble-like sales of a few titles – we have to wonder what would happen if in 3 years this bubble were to pop.
For example, will Jump’s One Piece, Naruto and Bleach continue for 3 years? We can’t really predict what will happen to them (and even if the publisher wants to continue them, the authors aren’t always able to).
The recent conclusion of the hugely popular Full Metal Alchemist brings this issue sharply into focus – not all series can be expected to continue indefinitely (Naruto in particular appears to have an eventual ending planned), and blockbuster successors for those which conclude have so far been conspicuously absent.
Other mangaka have complained the industry is not developing new talent – a particularly precarious state of affairs when considered in conjunction with reliance on a few blockbuster titles which could end with no replacement in sight.[/QUOTE]
This situation is going to affect the anime industry because a lot of anime are manga adaptations in the first place as well...