I say the series got let down in 64 because Kirby gets shoved up with Mario, Link and Pikachu as one of Smash's four "mascots" rather than DK, the unequivocal bigger character, as well as that Dedede was planned before Diddy, again, despite DK's series being unequivocally bigger. Those can both be chalked up to Sakurai and HAL bias. I also say the DK series got let down in Brawl because, after it got screwed in Melee, it got put into a position of forever making up for lost content, rather than keeping up with other franchises, and unlike Kirby, its creator isn't developing the game and adding in multiple characters in one sitting.
Ah, it's the "Sakurai is biased for his own characters" conspiracy again.
Y'know, despite not adding a single other Kirby character outside the pink puff himself until Brawl (and by then both characters added were major requests; in comparison it wasn't until
after Diddy showed up that any other DK character got any headway into Smash popularity).
Yeah, Dedede was planned for Smash 64, but unlike Bowser and Mewtwo, he was kept out of Melee for reasoning that doesn't have to do with not having time or resources as was the case before.
And so what about Diddy not being planned for 64? Nintendo had no ownership of him during that period; Rare did. Why do you think DK Jr. was brought back from pre-Rare DK for Mario Tennis instead of Diddy? Or that absolutely
nothing from Rare's DKC other than DK's design had been featured in Mario spin-offs until Mario Golf: Toadstool Tour? Smash 64 and Melee are literally the only exceptions despite not being Mario spin-offs. And even then, it was just locales, Barrel Cannons, Klap Traps, Cranky's silhouette, and trophies for Dixie and K. Rool (yet strangely
not Diddy...).
Hell, Sakurai even lists Diddy Kong as a Rare property on his pre-Melee poll. So if you want to point fingers at
someone point them at Rare.
Aside from that, Kirby didn't get pushed as a "mascot" for Smash until....Brawl, actually.
Each of the "Original 8" were treated equal in 64 aside from Mario, who was somewhat on a higher pedestal. But given that he's
Mario, "Mr. Nintendo" himself, it's excusable. Same with Melee; each of the starting 14 were treated equal except for Mario once again being on a slightly higher pedestal. And arguably Bowser too, being Mario's adversary and the most famous Nintendo villain. I mean, why
else would Sakurai create a monstrous version of him to serve as a difficult NPC opponent?
Then Brawl came around and had Mario, Link, Kirby, and Pikachu as the "main four" of promotion.
However, it seems Smash 4 broke away from that and is reverting back to the "everyone is equal but Mario" ways, though it could also be argued that's its the "Original 8" sans Yoshi, as well as Bowser, Pit, and Villager.
Also, I've had the low priority semi-clone Dixie discussion with you before, and I think your proof for that came down to her data not being listed right after Diddy's on the disc, but considering no other former duo character was cut from their counterpart, there's no way of telling whether or not that means solo Dixie became the consideration based on precedent. Metroid got screwed in a different way from DK in that it got content, but it got awful content that was in no way representative of either the series as a whole or what its fans (the non-awful ones, at least) wanted.
There's only one other duo to go by, and they've
been a duo since their debut. Not exactly the same situation as an incoming duo that gets split up before their debut.
Aside from that, the whole thought of Dixie's Instance Slot Data not being after Diddy's if all she ever was intended to be was a tag-partner is entirely illogical. What point would Sakurai have to
move the Instance Slot data in-between Wolf and Snake?
And no, it wasn't moved because her data has been dummied out since "Pra_Mai" has been dummied out while still is in-between R.O.B. and Jigglypuff. Mewtwo, Roy, and Dr. Mario are all in-between Jigglypuff and Toon Link, yet they're dummied out. Toon Zelda and Toon Sheik are also dummied out yet between Toon Link and Wolf.
Also, while she has no IC Constant data for attacks, she notably has an IC Constant for "CaptureSize" (still trying to figure this out) that Diddy lacks and also lacks an IC Constant for Crawling unlike Diddy.
One would think that if she was solely meant to be Diddy's partner, she would have the Crawling constant by default like how all of Popo's and Nana's IC Constants are the exact same.