Those roles in battle are super specific, and aren't a key part of every mode either.
Never mind Mario can be equipped by a version of himself, which inherently shows how much it doesn't matter. We have absolutely zero evidence to suggest Spirits in any way disconfirm. Just flawed theories. There's at least some reason to believe AT's(who actually have a major role in battle, and we know Final Smashes don't matter as proven hard by Chrom as is) are very unlikely to get upgraded in the same game. Those are very major battle roles. You know what aren't? Spirits. They have no real effect on Classic. You can have the same Spirit of a character who is already playable. They are not in the same league as AT's, and are more like Final Smashes at this point instead, which as we know, do not disconfirm.
The counterarguments are rather weak when they're still glorified Trophies that are slightly more useful than in previous games. They just combined 'em with Custom Equipment, but don't play any bigger role than Trophies either. You could literally just make them "equippable effect" without being tied to the character and they'd be exactly the same thing, just without pretty pictures. Really, the saddest parts about Spirits is they have no character descriptions, already making them a major step down from Trophies.
Also, being alternate versions of themselves doesn't make them any less the character. It just furthers proves they can easily have a different version as playable. That's a huge point in the favor of Spirits being upgraded, not the other way around. If we had no other Mario spirits, there would be a point to consider. But we do. Besides the fact that Metal Mario was once a full-fledged boss character in the first game too. Sakurai doesn't give a rat's ass about this dumb idea that alternate versions of characters aren't allowed to be playable. Now, you might have a point by noting that evolved spirits might be treated as a somewhat bigger role(because they have something very special for how they're summoned/used. That means they were intentionally chosen as a bigger role, not a fun quick thing to add), but the bunch of random spirits? Not even close. The only thing that really hurts characters like Shadow and Waluigi is their major gameplay role called Assist Trophies. Spirits are not even close to that level. Not even evolved Spirits are as close, though they're at least remotely notable.
Likewise, there's well over 1000 spirits. Why would Sakurai say no to a playable character because they have a literal png. That's pretty illogical. If a character won't be playable, it'll be for actual good reasons, like he doesn't think they'll sell well enough, it wasn't on a list Nintendo made, he has no interest in the character(admittedly this isn't that logical, but it does sound more realistic), some kind of character bias(again, it's realistic. He's just a human being), being unable to make the gameplay work right for a character(this kept Dixie out of Brawl, and may be why she has yet to be playable, if he wants to keep the tag-team dynamic), or if it's a 3rd party, they wouldn't agree to the idea. Maybe one or two of these are arbitrary. But you know what is entirely arbitrary? They have a simple png. That's a poor reasoning and isn't even worth his time to worry about. It takes almost nothing to rename the character in how it shows up, to say a slightly different name. And he only has to do this for one image. It takes more to make a character playable than it does to fix a spirit. That's the huge difference here in changes.