This is a false equivalency. As I said before (and as you mention later in your own response), I'm proposing that the ability is intrinsic to the inherent Cognitive Physiology of Personas (and by extension their users). Specific spells, such as Megidoloan, are not able to be utilized necessarily by any Persona user. Likewise, specialized abilities like Adachi or Akechi's Shadow manipulation aren't either, and have justifications in-lore for why their users have the ability.
Fusion Spell's function much like Other spell's in the fact that they are selected to be used via A Persona, or in this case, via multiple Persona. There is no reason to believe that Fusion Spell's are far removed from attacks like Megidolaon; Both are used in similar way's in game, and both are specific to certain Persona's. There are more cases where character's are able to use Megidolaon, too; Naoto and Akechi learn it, any Protag can use it, Elizabeth Uses it, Adachi Uses it if I recall correct in his boss fight, literally half of the P3 Cast use it in The Answer, etc. There is arguably more evidence that a move such as Megidolaon is more wide spread than Fusion Spells. And why exactly would Fusion Spell's be inherently intrinsic to the physiology of Persona's? Is there any evidence of that being the case? In order for things like Cognitive Phisiology to have been passed for Persona's, there had to multiple scans supporting it, correct? I would hope that at least one scan could be produced by the supportive side of the argument in this case that implies that Persona's would inherently have Fusion Spell's as part of their Physiology.
Tl;dr, There are as many or more cases where many persona User's can use moves like Megidolaon than there are cases of different Persona User's using Fusion Spell's, so the argument that said spell's are not used by many character's seems like a... False, Inequivalence? That's not a term but, the point is that it seems like a inaccurate assessment, as the two actually share more similarities than Fusion Spell's would with the inherent abilities all Persona's have.
I'm unsure what exactly you mean by this. Many of the abilities afforded from the page aren't explicitly stated to be beheld by every Demon or Shadow, but are rather logically extrapolated from multiple instances of different Demons using said abilities. One such example is cloning. There are no quotes or statements that indubitably or directly confirm that all Demons and Shadows can clone themselves, however, there are numerous examples of various Demons throughout different games displaying the ability to duplicate.
That's vastly different, as there is little evidence to say that Demon's wouldn't have such a ability. I'm not a SMT expert specifically, so bear with me, but if I recall correctly, even the weakest and least haxy Demons show occasions they can multiply, correct? Inversely, very few Persona User's actually show Fusion Spell's; Only the Persona 2 cast, the manga cast, and P3 MC are capable of them from what we can see, with a few other dubious cases one could debate. SMT Demon's and Persona Shadow' show time and time again, over and over and with a much vaster array of Beings the ability to clone. Of course, I could be partially wrong as, again, I do not proclaim to be a greater SMT fan, only a Persona Fan, but it seems that one has a larger array of instances that lend credence than the other; Neither the remaining P3 cast outside the MC, the P1 cast, P5 Cast, and likely P4 cast show this ability; I also believe that the Trinity Soul cast do not show Fusion Spell's. The vast majority of Character's do not show the capability, nor are they implied to have the capability, of having Fusion Spell's as a option in combat.
We can use the same line of logic I described before to determine whether or not the ability is intrinsic. From the cast of the
Persona 2 duology, to the
protagonist of
Persona 3, and even
spin-off characters, Persona users, regardless of the context or setting, have displayed the ability to perform fusion spells. There's no evidence to suggest such an ability should be evaluated individually, nor or there any (outside of gameplay mechanics) that imply it's relegated to specific Persona users only (just look at the Tsumi to Batsu scan, where normal Persona users are seen utilizing fusion spells without any specific explanation in regards to how they're able to do so).
EDIT: Fixed the link to the manga scan.
I am regretful to say that I currently cannot look at the Spin Off Scan, the only instance I am unfamiliar with, due to the Wifi network I am presently using, and for that I apologize. However, following this logic, outside of Gameplay Mechanics, what implies Megidolaon cannot be used by every Persona User outside of Game Mechanics? Very little, from what I know. In fact, there are many instance's of many Persona User's, weather Highly Adaptable (The Protag's), A unusal oddity (Adachi or Akechi), and even Normal Persona User's (Naoto, P3 Cast) being capable of using Megidolaon. There is very little difference between the two examples outside of one requiring Two Persona's. And that's another thing- How do we even know it's Fusionism? It seems more like the two Persona's tend to use their skills together and at the same time to produce a greater effect, but that doesn't inherently need Fusionism- Character's like Naruto and Sasuke from the hit Naruto Franchise are shown able to perform team attack's that combine their skills/abilities, but are not called fusionism; Most Fusion spell's seem much the same, not necessarily fusing their abilities rather than combining their attack's/skills to produce greater results. If this can be called fusionism, then technically, shouldn't every character get Fusionism, as they can combine their abilities together to produce heightened results?
I know this may sound like multiple false equivalencies and dramatic over the top examples, but that's indicitive of how ludicrous I find giving all of them Fusionism is.
