Spring Man was the poster boy for ARMS for sure and he was made the assist trophy, no doubt, for that reason. But we don't know what the future of ARMS is right now. The comic book was suppose to come out AGES ago and delve deeper into Spring Man's lore. But the comic book never came, but as of this January Dark Horse has said that the book is still in production. It's possible the whole thing has been rebooted and the story changed. Min Min could very well get a big push as a result of winning the party crash bash. In fact, Twintelle may get a big push as her popularity sprung purely from her design, the ads barely had much fanfare for her, other than some savvy screenshots. She was made available later in the Global Online Test Punch for the game as a result of that. It wasn't in the original plan.
We'll see what happens. As others have said, Nintendo decided to try and control speculation this time around and are having us ponder who the ARMS fighter could be. Under pre-release conditions, Spring Man would've been the obvious choice. Right now, we don't know what Nintendo has in mind for ARMS as an IP. I imagine the chosen character, whoever it may be, will be used in a bigger push for the ARMS IP lore and/or
marketability.
Pikachu is Pokemon's mascot today, but everything we know and love about the little rodent came from the Pokemon Anime (including the infamous 'say your own name' voice), which came after the release of Red/Blue/Green.