Not to jump in on this debate in any major way (because I honestly don't care enough to argue about it), but I figured I'd give my opinion...
Spring Man isn't necessarily the main character of ARMS, just as Tracer isn't the main character of Overwatch, I don't think anyone realistically argues either of those points. Like others have said, these are games based on the characters more than a single driven narrative, even if one is present, so at the end of the day, no one character is any more important than another, and no one character is necessarily any more representative of the series than another.
With that being said, like Tracer, Spring Man has primarily been used as the "poster-boy" for ARMS, and while not the main character (as there really isn't one), tends to see most of the spotlight within marketing/brand-identity, and hell somewhat in the game itself (with him being the default for the tutorial as well as the existence of Springtron).
Not knocking the plausibility of Min-Min or Twintelle or any other popular ARMS character getting in, but as the first representative of the series withing Smash, it seems more than likely that they'd start with the character most closely associated with being the "main" character, even if there isn't one. Following that character, is when other choices based on popularity or some other factor would likely be added. For example how Pikachu obviously was the first Pokemon added, and now they can get away with including popular choices like Lucario or Greninja, who while may not be of any more significance than any other character in the series, is undoubtedly one of the more popular choices. It's also the same reason why if an Overwatch character were ever to be added, it would most definitely be Tracer before anyone else. The same reason why we got Ryu.
But who really knows only time will tell.