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.
The journey to find the Fishman Island was still good and genuine One Piece greatness imo. But since the start of the Hody Jones conflict, it already dropped in story quality. Too much unnecessary plot dragging for the sake of drama.
Imo the reintroduction of Law, the lore of Punk Hazard, Tenka Ichi Budokai in Dressrosa was also displayed One Piece's superior quality. But when involving the core battle against Doflamingo, suddenly the plot drags on and on, the arc and battle also bloated meaninglessly.
For Luffy, Oda wrote quantity over quality. Oda slap Luffy in so many dragging chapters eclipsing others, but none of his character highlight or development feel meaningful in long-term. His powers were also obsolete once each arc was over.
For other chars, shenanigans. For the core conflicts, many offscreen and switching perspective. For Luffy's final arc boss battle, character and plot inconsistencies (are you having fun, Kaidou? Lol), dragging the fight until the powers become meaningless, give the final clash of ultimate moves itself 3-4 chapters without accomplishing anything lol.
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Yeah imo One Piece will be much better if Oda doesn't try to focus on the quantity (characters, plot points, subplot points, conflicts and schemes, battles). Currently, all those are cluttered, dragging, not earned, no satisying payoffs.
Even when the characters keep increasing in quality, Oda should better be more focused on handling fewer but higher-quality aspects of his work (core plot points, core lore, core characters, core conflicts, core schemes, core battles). Choose quality over quantity at the core, the rest can be just wide and generic decoration.