Oh great, this again. Alright, it's time to put this terrible arguement in it's grave once and for all.
First off, there is nothing officially stating that a character being a spirit, an Assist Trophy, a boss, a Mii Figher Costume, or part of someone's moveset (a Final Smash, for example) cannot also be a fighter or anything else. While it is usually the case that characters who are designated as one of those are only used for that purpose, there is nothing set in stone that says they can't be anything else. After all, we have plenty of examples of this not being the case: Chrom is both still part of Robin's Final Smash despite also being an Echo Fighter. King K. Rool, Isabelle, and Chrom are fighers despite having Mii Fighter Costumes still in the game. Rathalos is both a boss in World of Light and some character's Classic Mode playthroughs, as well as an Assist Trophy.
Second, Spirits aren't the literal soul of a character. There are spirits for
yoga poses, for crying out loud! Y'know, abstract concepts that don't have a soul in any way, shape, or form? Do yoga poses have souls? Furthermore, there are three spirits for King K. Rool in different outfits. Are you telling me that they're three different characters? If different outfits/artwork is enough to be a different Spirit, couldn't they just use Geno's star form (titled
♪!?) as the basis for his Fighter Spirit instead? Or is that considered cheating?
Third, I don't know how long you've been around with regards to Smash speculation, but before Mewtwo and Lucas came back as DLC for Smash 4, A LOT of people considered trophies a death sentence for characters in the same way you and others consider spirits. Go back far enough, specifically around the release of Smash 4 on 3DS, and you'll find plenty of people who genuinely believed that a trophy meant no fighter. And to their credit, they kind of had a good reason to believe that.
In Melee, your fighter would start every fight as a trophy before losing the stand, in essence coming to life. In Brawl, a character's trophy was considered their body, and if they're touched by a living fighter while in their inert state, they would reawaken. While certain characters would be used for multiple trophies (Kirby for example used to have trophies showcasing all of his abilities, both from his home series and in Smash), it was generally accepted that these trophies were the characters souls, and since fighters have their own trophies, to already have a trophy in the game meant you couldn't also have a fighter trophy.
Spirits are much the same thing as trophies from prior Smash games. You may like to
think they're different because of their whispy, ethereal design, but they're not different at all. So if trophies in Smash aren't considered enough to prevent a character from becoming a fighter, then spirits also aren't enough to prevent a character from becoming a fighter. And let's be real, do you really think Sakurai is going to not include a character because someone on the dev team had already included him as a goddamn PNG File? "Well I
could include Geno as a DLC Fighter to please longtime fans, but the group of employees I got to work on the Spirits already used his PNG for a spirit, so I guess I can't do that. Too bad I don't have the authority to overrule their descision because I would've loved to include him, but oh well! better luck next time, I guess!"