I don't think anything went wrong with
Back Cover.
I think the fans (myself included), got our heads a-buzzin' with this and that theory or expectation, and when
BC didn't fulfill every wish and whim, that meant it was bad.
Even now I'm a bit disappointed there wasn't even a frame of the Keyblade War.
With the way
χ[chi] ended, it's fairly obvious the game didn't end service on its own terms. I mean hell, Agrabah didn't even get a real ending, and the main story wrapped up uncharacteristically quickly as the deadline approached, including multiple timeskips.
As much as I was excited for and enjoy the ending of
χ[chi], it's not a big stretch to say it was rushed.
That in mind,
Back Cover is likely Nomura's way of filling in what ended up on the cutting room floor thanks to
χ[chi]'s premature closure.
So
Back Cover came and gave us this:
1. The Foretellers as actual characters. Aside from Ava, we saw precious little of these guys in
χ[chi] and only now do they really feel like characters with motivations, strengths and weaknesses. Invi is still pretty weak, but I feel the rest of them were done justice.
2. A true introduction to the Master. Originally a character we only saw in a silent dream, and one you'd be forgiven for forgetting about,
Back Cover gifted us with what I think is the best character in the series so far. The Master's playful and unpredictable demeanor make him both lovable and more than a little bit ominous. Also Sugita's performance was 100/10 pls gib me more
3. Deeper connections to the modern era and
KHIII. This movie came out before the relaunch of
Uχ, where the story took a hard left into crazy town and started heavily involving characters from the modern era.
At that time, while we could scratch our heads about Luxu wielding No Name or Chirithy being a Dream Eater, we really didn't have much to go on as far as what
χ[chi] meant for the future of
KH
So as far as the
purpose of
Back Cover is concerned, I think it absolutely succeeded. It makes
χ[chi] a more complete and satisfying narrative.
Of course, given the untimely and permanent demise of the original
χ[chi], I wouldn't have complained if
BC were another three-hour movie if it meant fully adapting the
χ[chi] story instead of only having the intentionally-unfinished retelling of
Uχ.
If I were to critique
BC as a standalone work, obviously we have a disjointed narrative with no climax or payoff, with constant references to plot elements which aren't established or made clear within the work itself.
But given that
BC is a companion piece to a larger work, I feel that it really has to be viewed in that lens.
KH is no stranger to bizarre narrative delivery (though Square Enix is certainly capable of
much worse), and I wouldn't have chosen to tell this story in this way myself, but that's how it is.
So, for what
BC was and what it was
trying to do, I think it did a good job and I enjoyed it very much.