Assist Trophies are such a loaded topic and generally I've tried to avoid getting in too deep to those kind of conversations, but I'll admit more and more I'm seeing a possible opportunity for a promotion of some kind. Of course we only have one more character left for this opportunity to occur and if you asked me to put money on it, I'd say it isn't going to happen. Still, with how many barriers have been broken and assumptions have been rendered false I find it silly to feel too strongly about what truly disqualifies a character from being viable.
Would it really be hard for Nintendo to go back on an AT and just prevent them from showing up when the promoted character is on stage? Of course not. And hell, I'd argue the presence of Chrom as both a playable character and part of Robin's FS shows that, even if the circumstances were slightly different, the whole obsession with what role x or y character already plays in the game means more to us than it does to Sakurai and especially Nintendo. I'd wager Nintendo doesn't care one bit, which leaves it up to Sakurai to decide whether or not that's an option... who again, added Chrom as a playable character who can be finished off by Robin and Chrom and met with a pose featuring Chrom on the victory screen. You get the idea.
The more pressing question is just whether or not there's any individual AT who warrants being promoted right now. Which is of course debatable, it's not very creative of me to suggest but I do think Waluigi has a leg up over the competition due to his significant fan demand. But if some other motivation exists for other characters then I wouldn't completely count that out either. My consensus is that ATs can be promoted if they really wanted to, but I don't think they see any pressing need to do so right now outside of the arbitrary boundary breaking of "an AT promotion!!", which would definitely turn heads but shouldn't and likely wouldn't be the sole reason to do so.
Consider though, the existence of a working model in-game is pretty convenient if they ever did want to promote any of those characters in Ultimate. Going back to Chrom, I imagine that's why it was so easy to add him in the first place.