Unless you're considering the possibility of Min Min's replacement, then why she was chosen over Spring Man sounds pretty irrelevant. Other than that though...I dunno. I feel like ARMS did well enough to gain a sequel, and if I'm right, then she's going to be there. If not then she's 100% on the chopping block (or at least on extremely thin ice as a not well liked character from a series without legs), but I don't feel like that's the case.
That's a slight misunderstanding of what I said about Spring Man. It's implying Sakurai treated him as an important character too. That's it. Maybe he was one of the first considered, or almost made it in, but the other two edged out. Maybe his mascot status made him important for the trailer. We don't know. It just means he held some importance and that's it.
I think we'd overstating ARMS' success still. It's a successful series, but still a one-off. Its only other product got cancelled, and as I said, its support was very quickly over with. It wasn't huge by any means. The game did not have that much lasting power, and while the sales are good by all means, it didn't continue to accumulate that much more. It pretty much was dormant for nearly 5 years, and then became dormant after the Smash Promotion gave it a decent sales increase. The lack of lasting power doesn't help, nor does the support being 6 months alone help(nor did have a fair jump in sales justify more support. We also didn't suddenly see the Manga come back as a possibility with that reasonable jump in sales either, so we have no evidence it mattered that much). There's really no reason to believe Nintendo has high confidence in the series with the data we have. It could absolutely get a sequel... but there's no good evidence for that. That's what I'm basically getting at. It's in a poor position from the data we have. Being skeptical makes sense right now.
I also don't think a lack of a sequel means she's a 100% cut. That's kind of silly to go about it that way. It just means that her priority is not super high at best. Like, I can still see her easily over Wolf, Lucas, and Pichu, as they're reasonably low priority due to either being from a series who likely won't keep everyone(Pokemon), or from a Dormant and Finished Franchise respectively. She's also newer, and a more overall popular character(though not as popular to actually play against), but whether some annoyance at her poor balance matters is another story. Though if you solely meant that if she's not in the sequel, she won't be in Smash, I can see what you mean by that. I still don't think that would get her cut, but that reading makes more sense.
To be fair, the series has repeatedly seen fit to bring back everyone if possible. Being low priority mostly matters now because about a big chunk the roster is getting cut by necessity.
That and IC's were... chosen to for a Retro Rep spot(nobody else has gotten this treatment). Being he went through multiple characters to find one he finds the best, it's easy to see they're a bit special in this case. In addition, more important characters have been cut too. Or at least from way bigger series. The success of the game wasn't why they got in, after all.
ARMS' success is also not nearly as relevant as being made in the way that it got her in. Again, we don't know the actual reasons Nintendo chose her. If they even outright chose her(due to Yabuki's request). For all we know, they threw out the idea because Sakurai considered the franchise. We're missing tons of details. This is why it's hard to say what'll happen. There's not much reason to have confidence in her return with a massive lack of details.
I still think she's likely, even without a sequel, but there's not enough clear data to have hard confidence in her return. I could see her being a bit of a lower priority, but at some of the higher spots among them. Over IC's? Uh... no, they're a guarantee at this point. They have an actual niche others can't do. Min Min somewhat boasts this, but having a really unique moveset that doesn't follow traditional controls is a niche others also have. I don't think her moveset is replaceable or anything like that. It just means that alone isn't a good reason to strictly keep her when others can boast the same point. Being highly unique obviously isn't a guarantee(poor Mewtwo...).
(I do want her back too, but yeah, there's a lot of flaws with some reasons being thrown around. It's really important to remember that when comparing characters, to make sure the context of their situations is near identical, otherwise it misses a key element of the reasoning. Sometimes some are best to simply not compare, if they aren't even in a slightly similar situation. A better way to put it is that IC's and Min Min have two things in common; first party and a really unique moveset. Nothing else is overall truly similar).