We doing this again? Fine, I'll just reiterate my points that I've always used:
Based on the promotion of NPCs to playable like with Spirits, Spring Man being promoted in the ARMS guessing game like any other character, and Bomberman's Mii Costume being near-identical to his Assist Trophy, I don't think Sakurai or Nintendo care if a fighter is already an AT or not. However, the amount of ATs with a feasible chance of promotion is slim, and even then they have their own issues (Not enough content to support a Challenger's Pack, dormant series, lack of fan support, etc.).
There, I said it again. Please don't make me repeat it anymore...