But why? Bowser and Ganondorf being playable didn't make Giga Bowser and Ganon underwhelming. If anything having
and
as playable makes Sylux more underwhelming as a playable character than as a boss because he would be the THIRD Metroid Villain. Despite already having two playable Metroid Villains, we have NO Metroid or Earthbound bosses in the game, so he would be in a more unique role as a boss.
Is it? How do we know they don't have Spirits because they're planned to be bosses? Bosses are given their own unique spirits as well, you can only obtain them when they're beaten. Wouldn't be the first time they've done something like this, the same way Isaac and Shadow were hidden away initially, causing us all to think they're playable, only to be revealed as ATs at the very end. In fact for Shadow and Skull Kid, the earlier confirmations of Knuckles and the Moon caused us to conclude definitively they would be playable, and thus was not the case.
And honestly if those two are the picks, I'd rather just have all 3rd party DLC. I don't think the odds of ever getting a 3rd Mother character are very high, barring Ninten as a Ness echo, unless the series suddenly comes back from the dead. Dixie Kong and Elma would be probably the most feasible 1st Party DLC I care about at this point (although I also wouldn't mind a Rhythm Heaven Rep), since the rest of my picks were already confirmed as AT or are even more unlikely, and both of them have a glaring lack of music from the games they starred in and potential for a new stage.
As for Spirits, again, I view Spirits as the successor of Trophies. So Mewtwo and Lucas having trophies didn't stop them from being DLC. Spirits take even less work.