This is a pretty all-purpose type of excuse though? Like, you can slot this line of "there must be a/was a reason" argument in as a defense of pretty much any decision Oda makes. The argument of "just because the characters/story say X or hype Y doesn't mean the reader should expect anything" is also a sort of universal criticism dismissal tool, and the root problem is that it argues that readers should not buy into the excitement the story very clearly is trying to instill in them. Its saying to not engage with the storytelling at face value. Its saying that people who got excited at the prospect of Nami getting a cool moment based on her dramatic last stand last chapter were overeager and foolish, despite last weeks cliffhanger being explicitly crafted to make you think Nami was getting a big moment and that you should be excited about that. And oda wrote the chapter ending like that even though Big Mom was there, so if you say that the events of the latest chapter are "expected" because of Big Moms presence, you're pretty much leaving last chapters ending compltely meaningless at worst, shallow and performative at best. You end up with "well Nami wanted to beat Ulti and that counts for something even if she instantly failed miserably", which is a damp squib of a moment and not really what the former chapter promises at all, in comparison to "Nami made a defiant stand and then managed to eke out a win against Ulti".
It is a bit of an expansive all encompassing excuse.
My argument / reasoning for hype is: (some) Characters are unreliable (and biased) witnesses. Characters will hype up X character because they believe it and it does often reflect in story. But if they are part of a organisation or been working together for awhile they obviously become biased. Caesar and the fodder of PH hyping up the Cool Bros is my example. Even Bellamy. Not taking anything away from their actual strength, it’s just that the people around them believe they are strong/unbeatable. Only narration boxes are definitive when it comes to hype. And or multiple characters/sources hyping up X or Y characters.
My point with the Nami, Ulti, Big Mom development is not that readers should not be invested and expect Nami to make good on her declaration. My point is that just because characters state that they will do X, should not make other characters in the same conflict lose their agency. Big Mom also had her reasons to fight Ulti. It’s not about us or fan service, sometimes the story should flow regardless of expectations.
Also another point is that it is clearly intentional and Oda has a reason for doing it if the development happens the Next chapter. People make it seem like Oda and his editors are writing these developments on a whim
And oda wrote the chapter ending like that even though Big Mom was there, so if you say that the events of the latest chapter are "expected" because of Big Moms presence, you're pretty much leaving last chapters ending compltely meaningless at worst, shallow and performative at best.
That’s my point. The next chapter Big Mom is still there. Oda and by extension Nami made her declaration knowing Big Mom is there. Oda and Nami both know that Big Mom is involved in this conflict. We could argue that Oda wrote himself into a corner with Big Mom being there and potentially stealing Nami’s moment… but Big Mom actually did something that is in line with her character arc during Wano with “avenging” Tama