Probably easier to accept that oda's plan is not as airtight as many of us assumed and that the man himself can be susceptible to retcons and poor storytelling (sometimes). It's the best way to explain the whole "ehhh we don't know that the gomu fruit is the legendary fruit" boo-boo that's happening now (same for imu)
If you're seeking for answers why these events could enhance the story, I think you should wait because the story can go different ways, so it's premature to make a thematic analysis of Oda's personal take on these themes. What folks appreciate in this chapter is the sense of awe they get from reading it, or the pleasure for aesthetic and poetry, or simply the mechanical satisfaction of seeing important events unfold in a story. It's electrifying! Besides, these tropes are popular too…
What you can do for now is start thinking about these concepts in general beyond a superficial understandment -- researching them, reading mythology, etc -- so you'll have more baggage to digest the story. After all, it's inexorable that all interpretations are made from personal references, biases, morals and knowledge -- and ours are much different than Oda's. By the way, people love to point out all the mythological references Oda put in the story, but people like to think about them as mere easter eggs and cool visuals, and ignore that these things may have significance for the author beyond the surface. But that's the author's choice to place his work alongside the tradition where he wants it to belong.
Personally, I'm so used to the idea of Luffy being the spiritual successor of Joy Boy that it's the only way the story makes sense to me. It's not only all the foreshadowing and prophecies, but the storytelling itself. The narration boxes in One Piece are like an ancient poet (oh Muse!) singing the story of a great Hero or a great King, traditionally mystified to be of a higher order to justify his greatness, like it's a tautological correlation for old poets. That's aesthetic, metaphor, and the mythification of virtues – that's art! The craft is that Luffy sets out on his journey to become King, but a playful misdirection named Pirate King. Nevertheless, the more we discover about the quest of finding the One Piece and about the Ancient Kingdom, the more the Pirate King becomes the purpose of liberating the world. I understand that, mechanically, anyone can arrive at Laugh Tale and become Pirate King (and do nothing), but this is a story, so it also functions on a thematic level – and here we see the narrative fusion of the mechanical with the deontological, thus the Pirate King is the Just King. (That's why I like theories of Black Beard getting to Laugh Tale first, since even though he accomplishes the mechanical goal, he only becomes a false king.) Every arc of One Piece feels like a construction of the ideals that make Luffy the Just King – especially the stretch from Romance Dawn to Skypiea --, and the values that make him prevail over any villain in real battles of will. Ages before the timeskip, the language of the story portrayed Luffy as a light that liberates the world from darkness. Even sheer luck conspires for Luffy's success all the time – and that's regardless of the story calling it destiny or not, although characters comment on it every time. In the words of Woop Slap, "is it his dream or his destiny?". Meanwhile, the character many see as the final boss – Black Beard -- is always talking about fate since his introduction. And all of this is rather mythical, especially on a meta level thinking about our history and the tradition of Epic stories.
What's ingenious is that Luffy already does on his journey the mission that the Pirate King is meant to do after learning everything in Laugh Tale. When two separate things (Luffy and Pirate King) are thematically so equivalent to the point of being conceptually inseparable, and finally they crash together at the end despite the theoretical "odds" in-universe (Luffy finds the One Piece), this is the definition of Destiny. You can't measure it empirically, you can't see the forces of the universe acting, but it's the human abstraction of extreme "coincidences" and special "circunstances". The writing is just accepting this concept by speaking it out loud. Moreover, it's the self-awareness of the story – and also its moral, a devir of sorts.
At last, nothing of what I said negates other themes in the story, like freedom. I just feel like people don't know how to contextualize other themes and facts once fate comes to the table. Which leads me to my next point.
I don't see much real criticism other than strict distaste for this direction... which it totally valid, btw, nobody has to like it since enjoyment always comes from personal biases, and that's how it should be. I really want to emphasize that nobody is wrong for hating it, but we should acknowledge that this debate is a moral conundrum instead of a mere storytelling critique. The issue is that most of this stuff are meaningless concepts for most people nowadays, even inadequate and outdated, especially in the West – so some people feel like "what's the point of this shit?".
What people crave for in stories is for the stories to reinforce their cultural myths and ideals – to inspire them in their beliefs. And what could those values and myths be today? For example, from rags to riches, freedom (the blockbuster version of the principle), individualism (your success is only your own), equality (not only of 'state rights', but ontological equality), equality of opportunities (which is the theoretical "meritocracy"), scientism (even in fantasy), secularism, etc… and these principles are all good to me -- I'm not criticizing¹. Therefore, the next step is projecting these values into the story as much as possible to extract personal meaning.
Thus, while One Piece seemed """ambiguous""", readers could ignore the contradictions and still find those ideals in the story. However, now Joy Boy returned like a wrecking ball, which apparently crashes all of our modern principles. I mean, Luffy is special, priviledged, destined, and all of this is kinda religious/spiritual too, and there's even the possibility of reincarnation. It just feels like the antithesis of our morals and beliefs, right? Well yes, but actually no.
We can still talk about freedom and effort in One Piece, no doubt about it. We can also think about fate under different angles (for example, I like the concept of Karma). It's very clear that Will moves Destiny in One Piece – and there are old Wills at play in this story which shape destiny all the time. Besides, you can still find inspiration in the story, or just admire the craft. Don't forget that stories are metaphors, not reality. But, of course, a deeper analysis of all of this will depends on how Oda tells the story from now on.
¹ I'm also "westerner" (latino, actually), leftist, a science-dude, atheist, secular, etc.
So basically you're arguing that people align beliefs to what they want to see happen in those stories. Which may be true sometimes, and i've enjoyed stuff like LoTR because of its themes, but given how oda has always wrote the One Piece as something that many people have risked their lives for despite the costs, and having it all be thrown away because of how specific the prophecies does stink up the overall message that oda has, in no small part, being building up to (unless you think all the people chasing after the OP are foolish when they were locked from the quest to begin with).
Like even if the themes of destiny and whatnot are prevalent in one piece, the opposite that freedom and free will are also very prevalent ideas consistently developed since the start of the series (and not some made up preference some of us have), and having the series take a sharp dive into the former at the detriment of the latter does hurt your narrative at the end.
And how does luffy being the hero equate to him having all the symbols be granted to him on a platter? My statement wasn't about the criticism lobbied at this revelations, but how does all these sacred icons being given to luffy enriches the themes of luffy having to struggle to get there and/or how he always saves the day because of it.
It's one thing for luffy to be a spiritual successor to the joyboy title, its another to realize he has all the pieces of the ancient kingdom from the start.
The former is absolutely not served by the special fruit/hat/one-in-a-million haki/heritage and the latter doesn't need these blatant comparisons unless you're a 6 year old schoolkid (and even i don't think they're that daft not to see how luffy will, in fact, manage to save wano irregardless of whether he has a chosen one fruit to begin with).