I've read through this thread, well admittedly just the first page, and I still agree a lot with what the OP says. But there's good counter-points as well.
I don't think Sakurai necessarily hates DKC, or not care about it. I think he's shown the franchise quite some love. But it's apperant that outside of Brawl, he was never really all that much into expanding existant franchises of Smash Bros. And we're just quite unlucky as DKC fans that Diddy was never considered for Melee, or even 64 (given how big DKC was at that time it was a good possibility). Because the massive outcry for Diddy pushed K.Rool and Dixie support waaaay to the background, where it was only untill the Smash 4 ballot that the Smash development team and Sakurai knew about the immensive popularity of King K.Rool.
Before that, all the DKC fans rallied behind Diddy's inclusion. And I think that was the part where DKC fans where the most united. We wanted Diddy in Smash, as we felt he deserved it. There was no division between the supporters as with Smash 4 with the K.Rool and Dixie fanbases. This was probably also because Diddy was more seen in recent games around that time. But it's not like K.Rool and Dixie where completely absent because we got tons of DK games made by PAON; DK King of Swing, DK Jungle Climber (EVERY DKC fan should play this little gem btw!) and DK Barrel Blast... shame Nintendo lost them, because am sure many more DKC spin-offs would've been made by them, and they where the most loyal to the franchise out of any developer (counting even Rare).
Also the whole mindset behind the newcomers in Smash 4 was working against King K.Rool quite a bit. It was all about who's recent and who's a big 3rd party character with good ties to Nintendo. Nevertheless, the Kremlings where added in Smash Run as enemies despite them having new appearances on a 3DS game or anything and the Jungle Hijinx stage was real hype and very detailed and true to DKCR. Knowing that DKCR was obviously the biggest thing happening for the DKC franchise ever since Brawl, it's also kind of logical that Sakurai didn't really consider a DKC newcomer a top priority. King K.Rool was also absent from that game, as was Dixie. However, Dixie could've / should've been a newcomer because of DKC Tropical Freeze...
It's a bit hit or miss with Sakurai and his love and attention for DKC. The animations for the Kongs and K.Rool are superb and couldn't even have been replicated by Rare themselves (speaking of modern Rare), and their movesets are full of detail towards DKC elements. There's good stages, but all of them are jungle levels.. which isn't all that bad because all of Metroid stages are lava based, and all of StarFox stages are space based. It's honestly not all that bad.
I would like to remember you all that Zelda also hasn't had a proper newcomer ever since Toon Link in Brawl. They sort of cheated with this, making Zelda and Sheik separate in Smash 4, and re-adding Young Link and revamping the 3 main Zelda characters just enough for Ultimate to make them feel fresh and justifying the existence of 3 Link characters, but yeah the cast is pretty much the same as Melee's or Smash 4's, only with a extra small Link...
Star Fox also had Wolf balantly removed from Smash 4, and safe for DLC, MOTHER also lost Lucas.
I think it's just Sakurai not really feeling it much of a priority to add to older Nintendo franchises, and favors newcomers from new titles or 3rd party characters. This is why King K.Rool and Ridley are more exceptions to the current trend of newcomers than the norm, and why they are appriciated so much for their inclusion. Echoes made it possible for Dark Samus to be in, otherwise she definitely wouldn't have happened. Animal Crossing got Isabelle, but we can argue that Villager was kind of a late addition to Smash 4 as well, because he was straight up denied for Brawl due to not being "fighter material". Even Pikmin had Alph denied two times for his own character slot...
Just try to look at the overall pattern of things. Newcomers of older core Smash franchises just aren't a common thing, unless it's from the Mario, Pokemon or Fire Emblem franchises. And even Fire Emblem just got lucky because of Echoes (
) and DLC (
).
So really, it's just Mario and Pokemon who are sort of guranteed for a newcomer everytime. Maybe we can get newcomers from the DKC, Zelda, Kirby, StarFox or Wario franchises after they've included the most iconic 3rd parties? Am not sure at this point... the roster is already huge, and we might only see newcomers if they are easily to recreate out of current fighters, wether it's being a Echo or a semi-clone. I could see the likes of Dixie, Impa, Bandana Dee, Slippy Toad and some Wario character make it, but only if theyr'e semi-clones... Which might be the highly suspected 'Fighter Pass 2' for all we know?
I won't judge untill we see the last DLC character being added. I personally think the "neglect" of DKC is highly because Sakurai favors 3rd party additions now. But my opinion holds no higher water than any of yours here.