I wouldn't say it went downhill. But there have been some major changes associated with a drop in quality:
- Story becoming more Luffy centric
- Less focus on the other strawhats
- Strawhats personalities relying on tropes
- Focus on uninteresting side characters
- Bad pacing
Most of these are a result of the world getting way bigger. Harder to focus on the other strawhats in a world with emperors/admirals/Gorosei/Revolutionaries. Easier to focus on Chopper in Skypiea or Brook in Thriller Park than in Wano with Kaido/BM/Kidd/Law/Marco. This elevated the role of the captain and downgraded the rest of the crew. Oda spending less time on the strawhats meant their characters becoming worse. Bigger world meant more side characters which meant less time on each character and side characters being boring/shallow. It also meant it is getting harder to pace an arc. It is getting harder to finish an arc with so many plotlines.
Most of these problems are a result of Oda massively expanding the world between EL and Marineford. W7/EL introduced the admirals/revolutionaries/emperors. Marineford set the scale of an emperor crew which is essentially an army with many commanders, underlings, and allies. You can't go back to the times when an arc had the strawhats, the villain and his underlings, and 3/4 island specific characters. Most of the problems of Onepiece had its roots planted in arcs mostly considered as the story peaks in the community. But this also had its upsides: bigger and livelier world, some really cool side characters, bigger sense of scale and stakes.
Other problems like art getting worse or stories becoming more repetitive is a result of Oda's age and the story going for so long.
