My thing is why would disliking something like Ash losing the Kalos League lead to deleting the Ash-Greninja form from continuity? If anything wouldn't favoring the scenario of Ash winning that League make them biased to keeping the form that would have gotten the win around (or at least send it off without literally retconning it out of flashbacks)?
My Conspiracy for Ash-Greninja was always that TPC didn't like the idea of Ash's win coming down to a new mon's super form as the poster boy. Notice how the two wins that made headlines (Ash winning Alola and then Leon in the Coronation Series) put their final bouts down to Pikachu, whether directly or indirectly (the Tapu Koko battle right after the actual tournament), and the Galar closer was Pikachu vs a Charizard user who they made no secret about being an Ace (bonus points for "Leon" also being the name of the Indigo League Pikachu that cost Ash the OG run). A good deal of this assumption is because I'm just cynical with their refusal to let you forget Pikachu (or Charizard) for more than a week ever since Gen 6 happened in general (up to and including Horizons) and Greninja's popularity plus story role setting it up as a prime scapegoat for such an example.
My Conspiracy for Ash-Greninja was always that TPC didn't like the idea of Ash's win coming down to a new mon's super form as the poster boy. Notice how the two wins that made headlines (Ash winning Alola and then Leon in the Coronation Series) put their final bouts down to Pikachu, whether directly or indirectly (the Tapu Koko battle right after the actual tournament), and the Galar closer was Pikachu vs a Charizard user who they made no secret about being an Ace (bonus points for "Leon" also being the name of the Indigo League Pikachu that cost Ash the OG run). A good deal of this assumption is because I'm just cynical with their refusal to let you forget Pikachu (or Charizard) for more than a week ever since Gen 6 happened in general (up to and including Horizons) and Greninja's popularity plus story role setting it up as a prime scapegoat for such an example.