It is absolutely not fundamental, because it doesn’t actually progress the plot or change the characters in any meaningful way
Of course it changes a character in a meaningful way — Palpatine himself, from dead and gone to alive and threatening. And his being back IS the plot. It’s THE challenge of Episode IX.
“I am your father” had no set-up as well and, by your logic, makes ESB feel disjointed with ANH.
Not at all. First, even if Vader being Anakin hadn’t been decided when ANH was made, it’s still much clearer that there’s at least something more to the story than Obi-Wan first tells Luke—Obi-Wan looking visibly uncomfortable before he tells the lie, Owen telling Beru he’s afraid of Luke having too much of his father in him, etc. The ESB and TLJ reveals enrich what came before; the TROS reveal muddies it.
Second, “betrayed and murdered” is not framed as some huge, shocking revelation, and in fact it’s positioned at roughly the same point in Luke’s story that “my parents will come back for me” is positioned in Rey’s. “I am your father” is the shocking mid-point twist of the OT, just as “they sold you for drinking money” is for the ST. They come at roughly the same points in their respective trilogies, upending the assumptions each hero started with. Luke has the rest of his trilogy to figure out how to handle this one challenging revelation; Rey in effect has to come to terms with two, one after another.
That kind of upending is fine to do once in a three-part story; doing it multiple times with the exact same question for the exact same character within the same amount of story is just juvenile. It’s the sort of thing that gives comic books a reputation for convoluted long-term histories as new writers come in and mess with what their predecessors did, but stories with distinct beginnings, middles, and ends are supposed to be better than that.
Also, I endorse everything Knight of Kalee just said.
Palpatine is essentially dead until he rejuvenates himself with the dyad’s life energy
“Essentially dead”? Come on.
after that he is killed for good, fulfilling the prophecy once again.
Is it for good, though? There is absolutely nothing in the film itself that tells us why we should be confident this is the case.
And I see you’re fine with Luke failing to create the Jedi or stuff like that.
Huh? No I’m not. And I’m not sure what that has to do with this discussion.
Rey never killed Palpatine herself, she just reflected his lightning onto his face - he essentially committed suicide.
We could just as easily say that if Rey had swung when Palpatine wanted her to, it wouldn’t be her “killing Palpatine herself”; it would have been her lightsaber blade killing him. The tool is not the act, and does not change the intention or causality behind the act.
Palpatine has to be killed by another Force-sensitive in order for Sith essence transfer to even work in the first place. Why do you think he didn’t commit suicide or have a non-Force-sensitive kill him a long time ago?
Even if that’s how it was intended to work (which is not stated anywhere in the film), then presumably he could have transferred into one of the Knights of Ren. Or an able-bodied Exegol cultist (presumably at least some were Force-sensitive). Or had his underlings scour the galaxy for healthy young Force-sensitives.