When oda start to write one piece , manga power in conmercial sector is not strong as much as today . Of course , he was the one who want to announce his manga . After commercialization of series , one piece is getting famous and famous day by day . Just for that , editors entered unnecessary things to series by convincing oda to gain more money . Like supernova and warlord system . If you introduced something , you have to use these characters . I dont think so oda is happy to extend one piece series. While one piece is seen as a weekly series that will put their minds at ease for readers , it is seen as something that makes money by those who trade this business .
second reson is absolutely animation . Demon slayer jujutsu kaisen or etc have good story line ? Nah ? Just animation quality collecting more people attention for these series . I did not see any art side of toei works . If they had done same process like tue other animes ( 24 chapter or 20 chapter high quality and non repeating things in the anime ) , one piece would have collected more people from new generation . Just check out demon slayer season 2 animation art
One of the sale decreasing reason is that new generstion manga readers dont interest any story line thing . All of them want to see good and clear fight and no extension and repeating thing . Demon slayer manga almost last 75 80 chap composed of fight and one happy end chapter . But check one piece , to able to make fan service , edotors make oda create rooftop scene . And we know that , this is unnecessary move totaally . Or luffy run ten chapter to reach rooftop to start roof top one piece chapter 1000 . Rule is simple . You can be genius but Your talent is revealed as much as the quality of the people around you.
- bad animation company
- idiot editors
- long years Of fatigue
- changing era and changing new generation perspective