Dark Samus and Daisy were added over 10 years well after their base character's debut. Neither were a straight-out costume at any point either. They aren't an alt situation ignored. They were never real alts to begin with. Only Dark Pit was a straight-out alt to begin with, and then separated at some point(after he properly existed).
The situation here is closer to Castlevania, but we aren't getting any bonus characters, so Echoes won't happen. They are a Ness situation in how they work, combining abilities to make one character. That's already going to happen with ARMS, cause the ARMS are what matters as the core representation in Smash. The character abilities aren't all that unique in every case. Anybody can easily kick, do a comeback mechanic, jump multiple times, or use the Dragon ARM. Not everybody can teleport or float either, or use someone as a springboard. However, many moves can be inspired by others. Spring Man can use his hands to give himself a boost as a reference to Barq & Byte's springboard ability. Not all moves are created equal overall. In Smash, you have well over 20 moves. No individual has 20 moves in ARMS itself. There's less due to how the controls work. Small tidbits/references to others is already inevitable. This doesn't mean using the same animations either, as that's where the real individuality lies. Which makes sense why an alt situation is not likely. That doesn't mean a frankenstein moveset is unlikely however. It's already going to happen due to the ARMS being the most important thing for whoever is chosen, but a lot of the abilities are actually generic anyway for others to reference.
Alts =/= using the abilities of others without using the exact animations(these are very different arguments and why people still expect outright an ARMS character will use some moves of another one regardless of whether they also have a direct alt or not). While Ness' lack of animations were brought up, this isn't a very good point in itself either. PK Thunder is designed where it doesn't target a particular character and can hit different enemies without a particular pattern to it. In Smash, this is how it works, not being in a straight line and goes all over the place. PK Fire can hit some enemies far more damage than others. This is represented in Smash by it staying on the field for a while by doing lingering damage, to capitalize on a burning-like effect. You can retool moves to easily fit the settings regardless of animations. While Lucario is clearly based upon his anime abilities and fighting style, it doesn't use the exact same animations all the time, taking liberties. This is to be expected to make it work. As I noted before, Spring Man can easily kick. He won't kick identically to Min Min because they have different overall animations, but it'd still work practically in the same way anyway, since it's an easy to do move. If you want a good example of this, look at Isabelle, someone who doesn't actually have a canonical ability to do a lot of stuff Villager does. Pocket and the Party Popper she shares properly with Villager. She's never once used a Tool, but she has handed certain tools like Party Poppers and Glow Wands(they fall under Tool gameplay within the series) and is easy to believe she can use generic items like that. Other animal villagers literally use that kind of thing. That's what people mean by generic gameplay being interchangeable enough to make for an easy Ness situation.
...Let's just say it's not really easy to explain every detail, but a Ness situation beyond the ARMS is very likely due to certain moves being generic enough to work. Ness had more canonical abilities than any particular cast member of ARMS too, but a lot of those moves weren't plausible within Smash either, nor make for a coherent moveset. The importance is coherency in making it work, not how accurate it is. That's why Ganondorf never fully used anything but his non-canon broadsword as a proper part of his individual moveset(the custom specials and taunt work without disrupting his moveset flow for TP Ganondorf, but they don't work for OOT Ganondorf because his moveset is brute force. A sword designed more around fencing doesn't flow well). It's why Pikachu only resembles its anime appearance while not working similar to the games much at all. Skull Bash is a great example of having a very clear animation being ignored so it makes more sense in Smash. There's no rearing the head back. It's just sitting still in a stance and then flying off. The anime also did it differently from Smash and the games. This is the kind of animation liberties being talked about, and happens with most of the cast as is. Or abilities that aren't accurate(Monado Arts loosely resembles Xenoblade Chronicles' system but makes sense for a Smash moveset). Etc.
These are all really good points too. Plenty of Smash characters have served as some kind of hybrid to represent a larger part of their series than just themselves and to fill spots in their moveset that would otherwise be lacking or repetitive and uninspired. I apologize in advance for quoting something that I only really address in this first paragraph here as the rest of what follows is more of a general post about these issues with alts/Ness tier movesets/etc in relation to ARMS.
I guess my question then for the people arguing an alt situation would be upsetting for them would be: would you honestly rather have a Ness situation? That seems like it would be far more of a disservice than alts that have slight differences. Not to mention the unaddressed possibility that this ARMS character if it is a multiple characters in one situation could be the first instance of alts with significant differences that retain personality of each alt.
You wouldn't be wrong to accuse me of not being an ARMS fan. But I am impressed by the characters and art style of the game and my short time with it, while in the end leaving me uninterested, I really saw it as a game with incredible potential and an incredible and diverse cast despite general gameplay feeling relatively uniform. More so I am a fan of Smash tho and many series in Smash. I'll hone in here on Metroid. Samus' moveset itself only partially makes sense, Ridley feels so wrong in the air to me, ZSS ..actually this one seems pretty accurate I think, and then Dark Samus is an echo of a character who's moveset already only partially makes sense mixed with the fact that Dark Samus could easily be a fully unique character and is wholly unlike Samus apart from appearance. Yet despite that last bit and Dark Samus having been a huge want for me, not only am I totally fine with the echo, I would have been totally fine with just a visual alt let alone an alt that has differences would have made me estatic. Echo is of course great too. In DLC we probably don't have the luxury or possibilitily of 3 echoes or possibly even 1.
What it boils down to is not a difference in experience or opinion on ARMS itself but rather on Smash. People arguing for a singular hyper accurate character and I simply disagree on a fundamental level. My preference is content and I am happy with more characters who served as odes to their home series opposed to less who represent their series and/or characters as accurately as possible within the confines of Smash. I feel like the same people arguing for a singular ARMS character are the same arguing that Frisk could never be a fighter because of how important pacifism is in UT and I will never ever agree with that stance plain and simple it is just wrong imo. Smash already shows how important and cool it is to have characters who are odes to their series or backgrounds (R.O.B., Villager, Dark Samus, Duck Hunt, etc).
Basically if Sakurai asked me if I wanted Shadow the hedgehog but only as an alt for Sonic or no Shadow at all then I'd say "yeah give me that alt please!" Not "no way dude it's either echo or fully unique fighter or nothing for me man". It is the same with ARMS because this is all we get for Ultimate. It's possible in another Smash game that we would get multiple ARMS characters separately but for Ultimate we get this one spot so I want it to count not just be one character with a hybrid or highly repititive moveset.
If someone would like to show me a really cool moveset for a singular ARMS character who does not have a hybrid moveset I would love to look at that and that might help me better understand or get my head around how that could work well and the level of detail that you're fighting for. But yeah basically all this to ask the people arguing for a singular character these questions: what would the moveset look like to you and how does it compare to movesets involving alts or hybrid elements (aka Ness character), would you be ok with alts that are basically echoes allowing for more personality and unique animations even though this is unprecedented, and are you ok with Ness tier shenanigans with the moveset more or less than you are with alts and why?
Here's a few thoughts to wrap things up. Do I think Sakurai sees things the same way I do? No I don't. I think Sakurai would prefer that all these characters be echoes. Although I could still see Spring Man with Ribbon Girl alt since characters already have multiple jumps in Smash and a rage mechanic with just some animation differences to give off their differing personalities. I think it's most likely that Sakurai would do Spring Man or Min Min as singular fighters or maybe Spring Man with a Springtron alt now that all this discussion has gotten me to think more about what Sakurai would do. I'm honestly not sure what Nintendo would push for, but I think chances are they would trust Sakurai's creative direction meaning that precedent and Sakurai's thoughts are the most important factors for considering what likely will happen with the ARMS rep. However, it's possible Nintendo pressured Sakurai for more or Sakurai challenged himself to do some alts that are basically echoes or that Sakurai saw a way to have alts without losing personality in some way that we are not thinking of. Please remember that Sakurai said this pass is truly extra and that could mean a lot of different things but alts like this could be one. Again I now think a singular fighter or Spring Man/Springtron are the most likely, but other things definitely could happen and the singular fighter even if done in a Ness-like fashion is definitely not my preference. I don't think I'm going to keep talking about this as it's kind of pointless. None of us are going to change each other's minds and that's fine. We don't have to agree on everything. We will find out sometime this month who the ARMS rep(s) really is/are and regardless of who it ends up being, Geno has a great chance of happening in this pass too and that's what really matters. Someone mentioned their perfect pass earlier so I'll end with mine (in order of how I think they'd be revealed even though this isn't going to be the pass at all):
1. Spring Man/Ribbon Girl/Ninjara/Min Min as alts that are basically echoes
2. Crash with a Coco alt
3. Geno (with a Genette alt just kidding I just want a Geno's Pizza alt...alright for real just palette swaps)
4. Rayman (with Hoodlum Havoc power up alts and/or Raymesis and alts from Legends)
5. This spot has a lot of competition unlike other spots. If it weren't for Crash being heavily related to Sony I would probably say Ratchet & Clank, but since Crash is already in the pass and Isaac is kind of samey despite differing moveset potential to a lot of characters currently in the roster, I'll say Frisk (could have *spoiler* alt) because I just think they would bring so much new things to Smash both by being an Indie character and with endless moveset potential.
6. Master Chief (lots of alt options with all the different Spartan armor options, could have an ODST alt, or just palette swaps) - I also think Master Chief could have a really cool moveset and people just aren't thinking creatively when they say he'd be boring.