But therein lies the problem. Nintendo's probably under the mentality that since Fighter Spirits exist the don't need to make spirits for the fighters as it would be redundant or break the lore about spirits losing their bodies or something so naysayers will always point to that.
You're expecting Smash Bros to follow the "deepest lore"? Well, surprisingly it does, even with the Partner Pokemon. Spirits range to plenty of characters including variations of them, as Galeem's lazers fired into the afterlife, as well as across the cosmos of space and time. This was clarified by Sakurai himself during the Kirby talk about how the puffball survived.
So, it's easy to explain how "partner Pikachu", clearly wouldn't be the same character as the Pikachu in Smash. Lord knows there's plenty of Pikachus anyway.
That being said it doesn't have to be a one way street. Just because fighters can't become spirits doesn't mean spirits can't be fighters. Their goal is to regain their bodies back after all while fighters already have theirs
Like I said. Likely for Season 2 or onward. Not really looking likely for Season 1, espeically with the unfortunate timing of Matt Bozon pushing for Shantae, by which time the DLC was already picked assuredly.
And from a development perspective it seems kinda stupid for Sakurai's team to lock themselves out of potential characters just because of a spirit they made before DLC planning even started.
That is, unless they were given strict instructions on the matter of the DLC, which comes to mind. Let's not forget that Nintendo were the ones who chose the list of DLC that Sakurai could chose 5 from. And we already know from the Box Theory that they don't care about how hard we sleuth.
Just like the Grinch Leak neither side will give up banging their heads against a wall until Nintendo says so so all we can do is wait...
I mean, with there being no sign of a Pirhana Plant spirit, despite the game being datamined, you can't blame people for leaning toward the Spirits deconfirm theory. Granted, that's not even the main reason I have doubts for Shantae in Season 1, as I stated earlier,
Yes, we'll see, but right now, with the correlation of evidence present, I'm saving my hopes for Season 2 DLC in her favor, so either way, I can be pleasantly surprised.