I just wanted to say that is a great point about Luffy's character. I have argued before that the SHs don't necessarily change, it's the world that changes due to their influences.
However, Luffy can sometimes wear me out because the formula for his character doesn't change. I understand that he's supposed to have an unwavering belief in himself. When the manga has been going on since '97, I think people get tired of him acting the same way. After all One Piece did go on longer than Oda original
originally intended it to.
I think that´s a part of it. The other part is, since Oda is concentrating on many more characters, the Strawhats, even Luffy, get less time and opportunity to simply act and show traits within the context of the arc, so much so that Oda tends to condense stuff that otherwise would have occurred more naturally. But when he does that, it ends up being like him checking boxes rather than developing it in a satisfying way, and whatever Luffy does or says looks like a plot device or a cheap catch phrase.
For example, in Alabasta and before, we spent a lot of time with Vivi, so both the audience learned to be fond of her, and also the relationship crew and Vivi was natural. So the crew fighting till the end for her made a lot of sense and seemed natural.
Turn to Dressrosa, the relationship between Luffy and Rebecca is completely artificial and just exists to justify Luffy being invested.