You're right, of course.
And again, with Smash being as unpredictable as it has proven itself to be over the years, there really is no telling what could happen.
Which also means there is no point in serious speculation, as it could all be wrong and meaningless in the end.
In fact, it has happened countless times already, when characters like Joker or Terry made it into the game.
Characters that came completely out of left field, with no hints or popular demand.
So me personally, I just don't see the point in speculating something as unpredictable as Smash, that has gone against popular expectations countless times and will keep doing so, for as long as the series and its soul is alive.
Of course, Shantae being in Smash as a Spirit is a victory already. And it certainly cements her long-term success, after a rough start. : 3
However, actually playing as her in Smash, would be an experience that makes the whole thing feel complete.