While my point about "leftover money" still stand ( It's been one year and a half since Ultimate release , people still following Smash news are Nintendo fans and Smash fans, and less the general crowd) I doubt it'll be shill pick galore.
Due to the complicated current times we're having , Spring Man might have not been supposed to be Fighter 6 for all we know. Or maybe they didn't cared much about the order of things.
Now , I want to explain why I'm so confident in Spring Man :
ARMS is an arena fighter. And ARMS is focused on it's core gameplay , like any game like this. Now , ARMS main gameplay is pretty peculiar , and every character show how they all approach these mechanics. However , you need a character to explain these wild mechanics , hence why poster boy/girl are needed.
Not all fighting games have attacks with such high range and commitements , so you need your straightforward character that rely on the core basics on the game to teach you everything. However , these characters focus on fundamentals , and reflect perfectly the level of knowledge someone has on the game.
In the context of ARMS , the game , Spring Man may feel generic. But in the context of Smash , he absolutely isn't. His design is simple , but nobody can punch you from accross half of Final Destination from what I'm aware of. This is a pretty wild design choice , and is already very hard to balance , Sakurai said it himself. Give him too much range and not enough recovery frames , and he is busted , give him too many recovery frames , and every move he do is more commital than Ganondorf's up-tilt.
So , if instead of Spring Man , you pick someone else from the franchise , you have to respect the base core gameplay , on top of how this specific character approach this core fighting style. This would make balancing a nightmare... And would make people think that "yeah , that's how every character approach the fighting in this game" while it's not. It wouldn't represent the core of the Fighting style of ARMS.
"But he's an Assist Trophy , and a spirit , too !"
Well , Nintendo asked for an ARMS rep , and I doubt they bothered about that detail. An assist trophy is a a singular item out of like 60 , wich has a 1,5% of summoning the character you want. This isn't a traditional assist from a 3vs3 Fighting game , it's a funny item to provoke some chaos.
Furthermore , Chrom. Also , to show an example from a different game : Android 17 from FighterZ. Android 17 was a core part of Android 18 moveset , he represent almost all of her special moves. Remove 17 , and 18 pretty much has one super , one kienzan , and some okay normals. And despise that , Bandai Namco still decided to add Android 17 as a stand alone character.
"But they would prefer to pick a fan-favorite from ARMS , like Twintelle !"
I already addressed this point before , but : Sakurai can take some liberties , and add those fan-favorites as alts. (Also , for proportions issues : Morton is supposed to be 3 times bigger than Bower Jr. , yet he's the same size as Junior in Smash. He can tweak the height of characters , and he can find a solution for Twintelle.)
"But they could announce the main character of ARMS 2 instead !"
Yes , we could get a sequel. However , the core gameplay will remain the same , they will just build on top of it , making it even more complex. So you still need Spring Man to teach the basics to new and returning players. Yes , they will expend on the cast. And they could add ARMS 2 characters as alts ! (One of the biggest reasons I think we don't have more details)
"But the main character of ARMS 2 could be someone else !"
But why ? Why kick Spring Man out , and design someone else that will litteraly do the same job as him ? It would be like removing Ryu from Street Fighter 2 , and adding "Mr. Karate man" , that will do the same things as Ryu , but with a new design just to shake things up and nothing else , when they already established Ryu's name !
"But ARMS Flopped ! They might want to reboot some characters !"
ARMS didn't flopped. It sold more than 2 millions units , and for a new franchise , that's fantastic. The reason everyone think this : It released a week before Splatoon 2 , and got no updates. It wasn't because the characters were bland. It's because Nintendo made a fighting game focused on it's core gameplay , and didn't bothered to support it like they did with Splatoon and Smash.