Haven’t gotten around to One Piece (that episode count is… daunting), but I think I really know it’s done as soon as they have a ‘tournament arc’. Give up all pretense and just have them fight for the sake of fighting.
And then there’s bleach, where, oh look, he has a somewhat cool sword, oh it has a cooler form, oh there’s an even cooler form, oh now he has mask powers, but limited, oh wait, we were lying that wasn’t his real cool sword form… Ugh…







I say it’s generally a problem of long narratives, but some genres like comedy can get a pass since they don’t have to rely on growth and progression.
To the extent a story needs to develop, running a long time is likely to doom something.
Running a few books or a handful of seasons can work, but if a story has to evolve over decades…