I recall watching a video that kinda answers your question. (more focused on the anime industry as a whole)

In summary, production studios risk being bankrupt by attempting to be ambitious. You would rarely see an anime that's not an adaptation because it doesn't make sense from a business perspective. If you have a story that's already made + good periodic sales report = ongoing

Tired of unoriginal slice of life? fan service? repetitive and predictable story line? Well fuck you. If being unoriginal sells because it craters a particular niche, they'll saturate the hell out of it.

yeah the economical aspect is probably the cause after reading what you and joelmules commented.
something in me still feels, that there is more to it than pure economics, but regardless thats far too circumstancial to expect to be able to comment on.

Madoka wasn't an adaptation of anything, and it became phenomenal.

Ye, but you know who it was written by and have expectations.

There are other examples like code geass, psycho pass, and cowboy bebop, but thats not really the point.

bleach canceled

So the real point is how the key to a successful shounen is to take some random subject all the kiddies love(such as ninjas and pirates) and give them some story. Although, make the progression so slow and meaningless with the right amount of cliffhangers to make it become a big hit. When they come close to whatever goal, you somehow invert the story and add a stupid side story to add more meaningless stories.

Eventually the main story goal is never met and you keep expanding new characters to hoard new episodes just to get their backstory while the story progresses to go in the most random direction. Still somehow make it relevant to the main plot, but not enough to where you can add even more amount of episodes.

Things like this also requires a lot of care when it comes to fan service because this show is aimed towards children and neckbeards. While you're at it, throw in random spinoff episodes and movies that make no sense at all but it's fine because you built up a big enough fan base to not really give a shit.


TL;DR - One piece sucks

Ye, but I don't blame the artists though. They don't want to let go of their cash cow

I doubt the publishers / anime studios want them to end it either

dear god, if you wouldn't blame your mother for selling you into slavery because it was profitable idk what's wrong with you.
they are to blame 100%

spot on

If the authors are making something shit and it's still popular, who's fault is it really?

blame the fans mate

yes blame the loli for being raped by a fat molestor