For some reason, this came up in my head recently, particularly regarding the long-running shonen titles. Once upon a time, whenever a shonen anime title covered as much ground as it possibly could in anime form, the animators would merely end the series and everyone would have to continue by reading the manga. (even if they had fillers themselves, it would be much shorter arcs…) Then, once enough of the original manga series was covered if not ended, the animators may return to the series and animate the later arcs to further complete the title. These seem to be the examples of series like Saint Seiya and Kinnikuman: both extremely popular manga and anime for Jump, both ended before the manga, and both picked up again years later (much later in St. Seiya's case).
However, this doesn't seem the case in regards to many of the more recent anime series. Whenever the anime has caught up with the manga, then the animators keep it going, milking out for all its worth with what could be seasons and seasons of filler instead of just ending and waiting for more footage. Filler is what ended up killing Kenshin and nearly drove Naruto to the brink...heck, even Bleach was supposed to end with the Soul Society animated, but the animators and ratings chose to keep going....but instead of waiting for Kubo to get further into the Arrancar stuff, they just did the Bount for the last year. Even One Piece, as amazing of an epic it can be animated, just keeps going and going with internal filler, filler arcs and non-airings instead of Toei just waiting for Oda to go further ahead before making another arc.
The question then is this: why do these shonen series keep going on and doing this even if they catch up to the mangaka? They'll still be popular when the anime does return, so why put us through these fillers and such? Is it because of brand recognition and sales of stuff or is it just because they want to keep mining from the series as long as possible without real care for artistic merit? (sure I admit hardly anyone calls shonen "art", but it has its merits and good points)