What's the evidence? Sakurai's vague statements where he differentiated between spirits and fighters because they were a new concept introduced in Ultimate? Or do you mean the pattern of "there were no spirits playable in the first fighters pass so that means there will be no spirits playable in the second fighters pass"?
Neither of these two points are strong evidence for why we won't get a spirit promoted as DLC. The second fighters pass was decided after the base game was released, the main reasoning why people believed spirits couldn't be promoted for the first fighters pass is because it was decided during the base game development. The only things that are truly deconfirmed are the mii costumes and spirits that were released as DLC. Timing is the main argument for why spirits can't be playable, and not just because they are PNG images.
Timing is most certainly not my main argument for spirit upgrades.
It’s the overall package of a fighter pack, as well as precedence.
Many of the first party picks that folks have suggested aren’t a protagonist, whereas literally every single other DLC pick has been exactly that: the lead character of their respective game. In addition, the only two Nintendo-owned picks were from relevant games of the era, and they both happened to come from the same game.
Now, it’s typical that “side characters” don’t get added before their lead character in a given series.
But here’s the next part: The Challenger Packs more or less have all come with entirely new content. This means that there’s nothing that hasn’t already been in the game prior. While Fire Emblem Three Houses is definitely part of the Fire Emblem series (which has been in Smash for awhile) the individual title was nowhere to be found. Other series, like Kirby, Mario, or Zelda, do not have that same situation.
It is based on precedent, and precedent that is clear and provable, that these things occur. The protagonist or mascot is typically the one chosen and, to this point, we’ve only ever gotten 1st parties from relevant games of the era.
Could that change? Maybe. But I’m not quite sure that after 9 DLC newcomers, Sakurai wouldn’t suddenly change his entire approach and add side characters.
With Spirits, Three Houses was speculated to get somebody based on the lack of spirits in an event. Astral Chain and other series got the boot from speculation, so while this wasn’t really the point you made, I don’t really get why certain series are exempt from this disconfirmation when it seems pretty clear that spirits
have been saved for actual character releases.
It simply seems more like a lot of hypotheticals rather than “here’s a reason to expect this”. That’s what I’m waiting for, because believe me...I’d love to see Rex, or Rayman, or somebody else get upgraded. But I simply don’t believe they will based on precedent and evidence.