So basically, Spring Man becoming playable and showing that Assist Trophy characters are eligible for DLC will result in… fans getting excited because their favorite character will possibly have a second chance at being playable?

Certain Assist Trophies become disabled and unable to be summoned whenever certain conditions are met… most of them because said character already appears in the background, such as Knuckles in Green Hill Zone and the Majora's Mask Moon in Great Bay… and then you have Alucard, who doesn't show up in Wii Fit Studio, with the given reason being that the stage has a giant mirror (in other words, they went to trouble to disabling an Assist Trophy just for the sake of referencing vampire lore).

If Spring Man or any other Assist Trophy character were to become playable, they could always just disable the Assist Trophy version whenever the playable version is present. No headaches regarding any balancing, no fuss.