I actually don't think that One Piece has gone downhill. But, I do think it's shifted it's focus from where it used to be.
Every arc used to be more character focused. And we used to be almost guaranteed to get a new crewmember every arc, because that's the formula that got established. I think this is partially why so many people keep expecting characters like Carrot and Yamato to join the crew. In the past, if a character got a major focus, they would join the crew. That's just how the formula worked back then.
But, nowadays, I think the story is less character focused and more plot focused. Oda is more interested in writing about the larger world and the larger themes of the story. He's more focused on the bigger picture, rather than the details and the character moments we used to get.
You could even make the case that the story is more villain focused. Because, I do think that the "modern" One Piece villains are usually written as being more complex and layered than the "classic" One Piece villains. Most of the "modern" villains have a sympathetic backstory, and Oda goes out of the way to show how their original dreams were actually pretty understandable. The villain's dream's just got corrupted over time.
Instead of being focused on the Strawhats, the story is now focused on the larger plot going on all around the world. It's more interested in the various factors and events that lead into how the World became like this, rather than on the adventures and desires of the Strawhats, themselves.
There's definitely been trade offs to this. Before, we would get more time with our main characters, and more chances for them to interact together. But, the story was much more similar to other shonen action manga. Nowadays, I think the story is a lot more complex and layered. There's a lot more ideas to unpack and explore in each arc. The villains have more complex and nuanced motivations. But, we've kind of lost that character focus and interactions that made the first part of the story so charming, and that's a shame.
A lot of people are going to say this happened post-timeskip. And that's actually fair enough. It's the most obvious place to point out how the story changed. A lot of other stuff changed around that same time, so it's just very easy and "clean" to say that it all changed during the timeskip. Some people might even say this kicked into high gear around the time of the Nika reveal in Wano.
But, I actually think the change happened earlier than that.
This video goes into more detail about the idea, if you want to give it a watch.
They actually make a fair argument that the change from being character-focused to plot-focused originally happened during Thriller Bark. And even nails down the very scene where the entire story changed focus.
"Nothing happened."
Now, this is a all-time great scene, don't get me wrong. But, it does show a change in how Oda approaches the story. Before this point, you could make a argument that ALL the Strawhat's dreams were equally important. They ALL mattered and had to be seen through to completion.
But, during this scene, Zoro is basically willing to sacrifice his dream to save Luffy. He's willing to die, to make sure Luffy survives in order to see his dream come true. Narratively, this makes Luffy's dream the most important dream of them all. The story is no longer about ALL the Strawhats achieving their dreams together. It's now about all the Strawhats working together to make sure Luffy's dream can come true.
And it's kind of telling that almost immediately after Thriller Bark, the Strawhats are all separated, and the manga becomes the Luffy Show for a few arcs, basically until after the timeskip. This is no longer a ensemble story, really. This story is now about Luffy. Luffy is our main character, and everyone else is a member of his supporting cast.
So, I'm not totally convinced there's been a change for the worse. But, I do think there's been a noticeable change. I think we've traded character complexity for plot and world-building complexity. And I'm pretty sure that change has it's roots ALL the way back in Thriller Bark.
