Just because something has good over arching story doesn't necessarily mean the character's are any good. One Piece is better at world building than character development in my opinion. Each of Strawhat's have a tragic back story, but other than that and the Usopp incident, none of them have really changed through-out the course of nearly 700 chapters.
Luffy IS the same as he's always been, despite all trials and hardships, but the fandom love it. It's risky to suddenly give him a drastic personality change, as fans may not take too kindly to it. The character's themselves are stuck in a formula, because the formula works. They'll always have the same interactions with one another, i.e. Luffy eating all of the food and Sanji punishing him, because the formula works, and we know none of them are gonna die until they achieve their dream, because what'd be the point of their journey? But it's due to this formula the series is getting some what repetitive, in terms of both the main characters development and arc progression.
I'm not saying the Strawhat's are bad, hell, I love Franky, Zoro, and Sanji, but they have become a little stale playing on the same tropes for over 10 years.