The thing with the ARMS fighter is that, if it were Sakurai who decided, I’d say it’s up in the air. However, since it’s Nintendo...it only makes sense that it’s Spring Man. If they really want to breathe new life into ARMS, that’s the best move considering he’s the one almost everyone will immediately know as the “ARMS guy” since he’s front and centre on all the promotional material.
It’s not just that, though. In a fighting game, you want every fighter to be seen as equal. A character shouldn’t get special treatment over another that would lead to them being more favoured. If this fighter is anyone else except Spring Man it would completely violate that. If that were the case, it would make one character wildly more popular and used than any other character, which would throw off the balance that fighting games are supposed to have. With Spring Man, that’s not a problem. He’s the main character, so he already has more attention brought to him. That’s why he’s the one with the most content in Smash right now.
Secondly, even if ARMS 2 is on the way, the main character will probably still be Spring Man. Usually fighting game sequels just add more characters to the roster. The characters themselves and designs are easily one of the best parts of ARMS, they’re not gonna get rid of them by replacing anybody, especially Spring Man. That’d be like Ryu getting replaced by someone in Street Fighter 2 or somebody being added to Smash and being fighter number 1, making Mario 2. At most Spring Man will get a redesign, but he’ll still be the main character.