What's to say? She's easy to make an Echo and pretty much plays like Diddy with a tiny bit of variance. So we're talking Chrom level, an easy Echo.
Have you played the games? Name the variances.
They make each Kong distinct from each other. I guarantee you if you see a rundown of each Kong's moves, sprites and animations you'd be find very little overlap. Dixie doesn't grab like Diddy, Dixie doesn't throw like Diddy, doesn't run, jump or attack like Diddy, so have you played the games? How much of your judgement came down to a glance at each character? The only overlap is the gumball popgun. A tweak to the Up B isn't sufficient.
And that happened actually very fast. Dixie took a very very long time to actually have a non-partner-based thing. She wasn't seen as notable to be alone in comparison. Diddy is also the Luigi of the games. Dixie isn't that same role, and got sidelined fast. They're not comparable situations at this point. She clearly was not treated as nearly important.
Diddy, like the other Kongs is his own character. Never started out as Green DK, not as a palette swap, and didn't take a handful of games to become his own entity. He came out the gate as his own; the point of each Kong is to give a different gameplay style. Otherwise you have no reason to swap. You got more than movement speed out of a particular Kong; they attacked and interacted with the game in ways specific to them. He's best known as DK's partner yet still he stands alone just as easily. There's nothing stopping Dixie from getting the same treatment.
It's not contradictory. Making her an Echo next game is easier than spending more resources on her. If it's between that and partner it'd be up to Sakurai which he values more; the mechanic she was meant to have in the first place, or just adding her for the players? Being her upgraded Spirit is still a partner one, the development team behind the Spirits at least sees Partner as relevant to her character. Does Sakurai still? I see no evidence to suggest otherwise. All we know is he wanted Dixie to be a partner character and she has yet to show up outside of Trophies/Spirits/Stickers... and eventually has an evolution into a partner-based spirit. Both combined isn't saying much, but it does look a bit suspicious.
You're saying Transformation is too costly for the DLC cycle. Your solution to her being too resource intensive for one cycle is to save her for a new game, where there's greater resources. Then you say drop the Transformation altogether for the game that has more resources. "Let's save Dixie for the next game, since Transformation is too taxing. Let's make her an Echo for the next game if she's not fit for Transformation." There's no point in bringing up Transformation then, remember that you said it. That is a contradiction, and shifting goalposts. Whether she's meant to have it in the first place or Sakurai's trying to please players is irrelevant; the point of adding characters is usually to please players AND match Sakurai's vision.
Your next paragraphs say you doubt Echo is on the table for her, but if Transformation doesn't work then she'll be an Echo, which you don't think is feasible yet it's really easy to do. Even semi clone isn't an option, but somehow Chrom-Level is, despite you not thinking she'll be an Echo. This is contradictory. If Transformation doesn't work, and you've already built a moveset for her, then drop the Transformation and make her solo. You've already built the moveset, so why scrap it and go with Echo if you can just change the Down B at least?
I still think she's most likely a partner character, and the fact she didn't get an Echo spot(which is what they would've had time to do in Ultimate for her, possibly in place of Daisy, honestly. Since Daisy was just a quick one. Though I could see both at this point) is saying something. It doesn't in any way suggest she'd have a chance as a solo semi-clone either. There's absolutely zero things to suggest it ever had a chance. It was either easy development or "choice we want to do". The middle one has no real evidence or logic towards it. "Semi-clone would be nice" isn't based upon logic here.
I'm unconvinced she ever had a remote chance as a semi-clone(alone. Maybe as a partner to Diddy), and I doubt she would've ever been an Echo either(but at least there's a slight chance for it, since she would've had some time to do during development. But if they didn't, that makes it even worse. That she's lower priority even when ease of creation).
So overall, you're saying you think she'll be transformation aka partner in this case, but if that doesn't work make her an Echo, since solo is unlikely. Basically, whatever moveset they come up with should be scrapped in favor of tweaking Diddy's if transformation doesn't happen. At that point they should use whatever they came up with for her and fill in the Transformation parts with something else, which is a Down B at the bare minimum. No point in throwing out an entire set because another character couldn't come with her. That's like scrapping Dixie AND Diddy in Brawl because Transformation didn't work.
If you're still trying to convince me she wouldn't be an Echo, you're barking up the wrong tree. The only legit reason I've seen is she couldn't actually use a restructuring of DK's Up B, but that isn't even true. Which means she's easy as a Chrom-style Echo without issues. The rest have been purely based upon "I wouldn't want this" which has no meaning in what can happen. I'm basing this upon actual development, statements, and how the game works. So I only really see Echo(not that likely) or Partner(most likely) as realistic with the information we currently have.
Then you don't know how she plays. Her grab game is already something Diddy can't do with his tail. You need a Dixie who uses hair for attacks, grabs, movement and throws. This is more than a change in ground and air speed; her own sets of grabs and throws, her dash attack, her own taunts, run animation, jumps, something resembling DK's Up B or greater. That at the bare minimum is far more effort than Chrom; side by side the only difference he has with Roy is his Up B. But then again, you don't see Echo as likely but you're confident it's plan B if Transformation is an issue. If the Transformation/Partner aspect can't work, there's no point in scrapping everything and transferring a tweak of Diddy's moveset for a solo slot when the moveset can just be altered.