If Lucario can destroy the roots, why can't Alain's Charizard destroy it?From what was seen, only Greninja and Lucario could destroy these roots, so Alain's Charizard wouldn't be able to help.
Note also that only Greninja can detect these roots, so that another Pokémon cannot fulfill this task.
From what has been said, the only tricks available during PWC are G-max, Z-Move, and Mega Evolution.
Greninja does not have any of these (her ability is "Battle Bond"), so I don't think Ash-Greninja could be used.
As for the detection of the roots, that's an interesting point but they could definitely write around it. (Plus, they could even make it temporary. Something like Alain will hold down the fort until Ash's battle.(
Ash Greninja objectively is not a gimmick. It is a special Pokemon Ability. Something like Infernape's Blaze. In a way, it would be like Lucario's Aura boosted attack which was used.
Naturally, Ash will not resort to using any other gimmick in the same battle, but Ash Greninja is definitely fine to use in the PWC.