What is the moment, the arc, where One Piece really lost its charm and way as a story, and started to become the shadow of its former self it is today?
For me the Tamatebakko explosion is the point from which I started to become increasingly baffled and frustrated with the manga's story and writing of its characters, especially with the awful wedding cake chase and Luffy vs Katakuri, and other plot armor shenagans. I liked the first half of WCI well enough but despised the second half.
If not that then it was Wano arc with Oden's flashback, and especially the Onigashima raid with all of the horrible reasons we know.
Actually it began in east blue, gradually and nearly invisible but became worse slowly but surely.
This is the case with many stories, perhaps with most stories. What sets one piece apart from most other stories is its enormous length. As long as a story gets the chance to progress and find a natural end within a reasonable timeframe, the decline is barely noticeable.
My favourite example of this may be Avatar. Over the span of its 3 books, the series progresses and declines at the same time. While it's climax is obviously the fight against the Fire Lord, the most densely packed arc is book 2 but I'm almost willing to say that book 1 is the best arc because it serves as an exposition of the characters, their motivations and the world they live in.
Avatar feels like a constellation of stars or planets, slowly beginning to move into their places in book 1 and beginning to glow, reaching the constellation towards the end of book 2 with their brightness increasing, beginning to leave the constellation in book 3 but their light shining brighter now than ever before obscures this fact.
Only for the stars to finally part ways in the very end of book 3.
There is no way for the stars to return to the constellation, the comics feel like additional background material, Legend of Korra has near completely lost the spark that made avatar so great.
Had the creators been given the chance to make a book 4, I'm frankly not sure it would have lived up to expectations. It might have become stale, avatar might have quickly outstayed its welcome.
In hindsight, the series ended at just the right moment.
This is what One Piece is lacking, a reasonable time frame in which the story is told, a clear roadmap the author actually sticks to.
So many manga and anime eventually lose their spark or go nowhere, or even worse, are never finished.
The issue lies within the industry that prioritizes profits over a good story, audiences are tied to their IPs by means of fanservice and cheap&easy satisfaction through generating intense but empty emotions.