Ohk this maybe just my opinion but everyone manga does fall after reaching a peak. Literally every manga. Mha , aot , Naruto , sds , jjk. Just look at what happened to sds and Tokyo ghoul. People just lost their hype. Because author gave them a peak already and they set that as their benchmark. Marineford was peak mainly because though luffy being important there, it focused a lot more on side character. A complete masterpiece with everyone getting panels they deserve rather than wasting it on Mc again and again.
I only disagree with this because to me peak One Piece is actually Water 7 and Enies Lobby, i like it way more than Marineford
yet, i still enjoyed Marineford of course
so a peak didn't make the next arcs bad for me
you know what i mean?
I also still really liked Naruto after Pain, i only had problems with Kaguya, and even Naruto vs Sasuke after Kaguya was great for me
AoT for me was great right up to the last 9 ~ 7 chapters, and mainly the last 2 chapters
the rest of them i can agree
it's not a set rule, i think you can still make quality arcs even past a series peak
but One Piece felt like it changed, it wasn't a story about adventure with your crew anymore
became the Luffy show and Straw Hats became side characters, and Oda makes a bunch of side characters each arc that steal even more time from the Straw Hats
and the arcs don't feel like adventures anymore, at least not me
it felt like Oda trained his viewers to expect and enjoy something, and then took it away from his readers