But, Gregm surely, after decades of writing and drawing these characters, is it really that far-fetched to expect a certain level of, as you say, infallibility?
Definitely not. Of course, as Greg said absolute perfection will never be reached, but speaking of a certain level of writing quality it's perfectly fine to have certain expectations. There are measurements of quality in the department of writing of dramaturgy. That is fact, otherwise it would be impossible to compare two different pieces of work - like Hamlet and some story I've written like 25 years ago. I mean there's no objective way to measure it so it's especially impossible to determine whether one of the two is better or worse, eh?
! Bullshit. My story is garbage and fails on absolutely every literary level possible. I just had no real idea what I was doing. Only thing I can give me is that most of the time I've written somewhat systactically and semantically correct sentences. And even that with a big grain of salt.
There are lots of dramaturgic principles every kind of author has to follow which to a great deal have been always the same since milleniums.
And that's where the problem with present day Oda and One Piece lies: He ignores a lot of these basic principles.
One of these principles is that there is a dramatic question which gets established at the start and gets a resolution at the very end of a overall story, story arc or even subplot. Is it possible to answer that question already midway? Actually yes. it's perfectly fine - as long as you right away ask another new dramatic question which is equally hard to solve, or better even harder to solve, therefore creating even more tension and stakes.
Oda, however, nowadays instead of answering these questions properly he just replaces them by weaker easier-to-resolve questions and then answers these. That's basically literary strawmanning. He does so in almost every plot recently - be it main or subplot. No one expects Oda to be absolutely infallible and perfect, but in case of chickening out from previously established question the incidents where he fails aren't singularities any more, instead it's really basically every subplot. Surprisingly the Nami subplot is no exception.
What was the initially asked dramatic question from the Nami subplot? So the Strawhats arrived in order to take some big names. Not only did Nami call for Ulti, Ulti also accepted the call by going exclusively after Nami now. Not someone else, it was personal for Ulti now. It has to be Nami and no one else. So the dramatic question is:
(1) How will Nami survive and overcome that ridiculously strong veteran - and more ressourceful!!! - Yonko Commander that will go right after her head?
However, then Big Mom happened and we ended up with:
(2) How will Nami and friends overcome that almost passing out former shell of herself that lacks any ressources now, anyway?
That is the question that got answered. But that's also a question no one cared for even in the slightest.
How about instead that happened (as one of the many possible different outcomes):
When Nami lied to Big Mom, Ulti showed that she also can be decently (street) smart.
Ulti: "N-No... She lies, it was her who is lying. gulp A-Ah, see that Tama-girl. Right now, she's running away. sigh Because THEY are the liars. Nami is the one who beats up innocent children. Nami is the shit here!"
And then Big Mom goes after Nami, too. Being the 5-yo-child she mentally is she makes it some kind of competition with Ulti whom see sees as a rival child. So they both go after Nami who only barely survives because they also sabotage each other. (Luckily BM really rspects Ulti enough to not attack her even in the slightest.) Ulti's grin afterwards is priceless too, of course. So we get:
(2b) How will Nami survive and overcome Ulti AND fucking emperor Big Mom both exclusively going right after her and only her head?
During the following confrontation now only did she get separated from Usopp, but Ulti and Big Mom also got in some heavy hits. Despite none of the attacks connecting fully due to them sabotaging each other, Nami nevertheless is heavily injured and on the brink of death. (The zeus absorption also happens somewhere there I guess.)
Of course, at the end of the day Ulti doesn't want to share the kill with her rivalress. So she gets rid of Big Mom. Not in direct combat, of course, but by exploiting her 5-yo-child mindset and directing her attention to something else. (Big Mom is out of this and leaves towards one of the another subplots currently going on.)
Ulti again goes directly for Nami. However, Tama steps in in order to buy some time. She attacks Ulti with dozens of her tamed creatures.
Ulti: "All you can do is run and exploit others by letting them do your dirty work? You're pathetic. Claiming to care for children, but then just let them fight for you to save your sorry ass, completely fine with them getting slaughtered in battle. If you think you're the better person, you indeed are the better liar out of us two!"
Nami, heavily injured, only barely can crawl away. Then Ulti appears right in front of her - dropping away the beaten-up "dead" Tama to the side.
Ulti: "Now it's just you and me!"
Ulti grins even more devilish than she has done before all the time.
Ulti: "But I somewhat like you! You can become my subordinate. Just quit Luffy and the Strawhats! I'll only have to replace your silly tattoo by our sign, then you're in. Really. No big deal!"
Of course, Nami refuses. Thanks to Zeus she barely manage to escape Ulti's clutches. But still, even combined with Zeus' powers she still cannot inflict any damage at all to Ulti. Nami tries to hide, whereas Ulti continuously charges for her.
Ulti: "Attacks such weak don't work on me!"
Nami: ("Nah, that's not what happened. You perfectly manage to ground yourself instinctively. Just like Luffy, you indeed are some battle genius!")
Therefore we come to (with counting Zeus as part of her arsenal now):
(3) How will Nami survive, overcome and finally beat that ridiculously strong and more resourceful veteran Yonko Commander that is also super battle smart and now even more bloodlusted to go right for her and only her head - with no help in sight in an unavoidable 1 vs. 1?
During the course of these plots and subplots I want to have added more obstacles hindering the protagonist instead of them getting removed, leading to a weaker strawman dramatic question who no one cares for at all. I want to FEEL all of Nami's pain and suffering, so that I can genuinely root for Nami to finally overcome all that insurmountable mess after the hardest never-seen-before struggle in her entire life up to this point. But that's just not the case here. Whatever happens to Nami now, she'll get through it without any real difficulty. So why should I care and root for her, anyway?