Using the fact that alt costumes/skins weren’t in the demo is a fallacy in itself.
For a more obvious example, Meta Ridley was shown in the trailer, but was not available in the game. Does this mean the Meta Ridley costume isn’t in Smash? Of course not. It just wasn’t in the demo.
Likewise, Mario didn’t have his Fire Mario costume in the demo. Does that mean Fire Mario isn’t in the game? Again, of course not. It is, it just wasn’t in the demo.
This also applies to Assist Trophies. Considering Knuckles functions differently from Shadow, there’s no reason to think Shadow was replaced by Knuckles when they could easily coexist.
In Wii U, there was no sign of Isaac as an Assist trophy. If it was assumed that Isaac was playable, those assuming that he was playable were unfortunately wrong.
It’s waaaaaaay too earlier to make these calls. If Robin has a different final smash, then I think there’s a reasonable chance of Chrom being playable. The absence of the color scheme is not a reason to expect Chrom because it could still easily be in the game.
Likewise, Ashley, Shadow, Isabelle, and Skull Kid not being in the demo means effectively nothing at this point because we don’t know at least 20-25 Assist Trophies or something like that.
Frankly, saying something that’s missing from the demo confirms something is like saying that any character not on the roster in the demo is not playable. That’s nonsense, especially because we KNOW there is more content. There’s no reason to think differently with Assist Trophies, Stages, Music, Items, and costumes.