At this point Spirits still most likely deconfirm, considering the rule only got amplified with each update instead of getting a hit.
The core of spirit theory is not the artwork, but how Spirits are named in the first place.
People love to bring up the fact that there where plenty of alternate spirits in the basegame but all these spirits are named differently.
The real Link is called "Link" and thats his fighter spirit and other Link spirits are named after the Link from that game (For example "Link (Ocarina of Time)"
The Link rule also applies to all the other characters like Samus, Wii Fit Trainer and King K.rool who also have alternate spirits, and it even goes as far that if an alternate form is a costume option like Meta Ridley, Koopalings etc. they are labeled as Fighter spirits to (for example if "Kaptain K.rool would have been a costume, it would also be considerd a Fighter spirit, and DLC has amplified this rule anyway with Joker's student costume and the 4 Dragon Quest heroes, following the same ruling)
Also my Link example got amplified by the Lets go spirits and the Three Houses spirits, in which once again they named the new Pikachu spirit "Partner Pikachu" to differ it from the "Pikachu" Fighter spirit and while the Three Houses update did show that they are willing to change the naming of a spirit, the only thing they changed was the Hilda and Dimitri spirit, to differ it from the base game spirits of characters that share the same name and come from a different franchise.
So yeah the only thing to shatter this theory is if we get a spirited character and the fighter spirit either shares the name of the base game spirit or they made changes to either the base or the fighter spirit to differ those spirits from each other.
For example lets take Rex. Both spirits need to be called "Rex" or Sakurai changes one of them to "Rex & Pyra"
In this scenario Rex still has a chance(but he has another hurdle to overcome which will be my next point) because, calling his fighter spirit "Rex & Pyra" keeps the whole theory intact still and the vanilla "Rex" spirit then would act the same way as other alternate form spirits, but I feel other characters are less lucky like Rayman for example, because then both spirits once again need to be called "Rayman"(can't really apply the Pyra trick to him, unless they change the basegame Rayman spirit to what game he comes from), so while its offcourse not impossible, you need to realise this is the scenario that needs to happen to make a spirited character playable.
Pokemon is also the only franchise I can see breaking the Event spirit rule, because Pokemon is a rulebreaker in general and the most likely Gen 8 reps aren't spirits yet and while it did have an event, they can still draw enough spirits for their spirit board by using the upcoming SwSh DLC as source material, which is what I would consider a problem for some characters to (this is the hurdle for Rex I mentioned in my previous example, because Xenoblade 2 already got its core stuff as basegame spirits)
To give you an example how easy its still is to draft enough material for the Gen 8 spirit board:
Cinderace as the fighter spirit
Rillaboom, Intellion, Wooloo, Toxitricity, Kubfu(with Urshifu being its upgradedable form) and you can fill up the rest with Dynamax forms for example.