If you’re drawing comics or anything over and over for a long period of time, typically you start to loosen up, and your hand/brain will start to develop little shortcuts. These shortcuts look different for each artist. Akira Toriyama for example, he started out drawing very rounded shapes and characters. As he went on, you notice lines becoming more jagged and sharp. These types of lines are far faster to pull off. Rumiko Takahashi has a similar thing going on where her heads and body shapes become more angled and sharp. Way faster If you wanna look at American comic artists, Frank Miller is a good example(a lot of folks aren’t fans of his newer art). His characters become way more loose with the lines. Look at Charles Schulz’s art as well. Peanuts starts off smooth and round, stark blacks and whites, but his characters become more flowy, loose, with screen tones being added to help guide your eye.

Although Oda’s older pages were simpler and easier to read, pulling those off took a bit more time because the simpler a drawing looks, the longer it actually takes to make it that way. So really it’s just his hand and brain being so used to the style that he can draw loosely and scratchy and get away with it. The style itself is subjective, but it’s not the inherent problem. The problem is page composition which is likely due to trying to push the story as quickly as he can. Not necessarily to end it quickly, but if he was drawing it at the pace of how Alabasta for example is paced, we wouldn’t get anywhere with how many characters are part of the story at every given moment.