This. A lot of people think Ash-greninja is the equivalent of doping or using drugs to boost your performance which is utter nonsense.
Here's the problem i've always had with Ash-Greninja, since the writers couldn't swallow their pride and allow ash to get a real mega, they decide to enter fanfiction territory and give Greninja a transformation that supposedly is to be on par with it.
the problem is they went to far, and it overshadowed the fact Ash finally got a fully evolved water starter, making it seem like an afterthought for one thing, but then they had to make it crutch based, making one wonder just how powerful Greninja is without it.
You need to rewatch XY cuz they explicitly told us that AG was a result of Ash and Greninja's bond and helped unlock his true potential. It's not the same as mega evolution, dynamax/gmax and z moves.
When power becomes a crutch that you have to rely on to be powerful and not an extension of your abilities, that's not a good thing.
Greninja hardly got the chance to prove itself as a Greninja, almost every fight it was involved in after becoming one had it's transformation triggered, and the one time Ash tried to force the transformation it backfired and he lost, so how can we truly discern how powerful it really is?
at least with pokemon going mega, that's an extension of their already established strength which with one exception, (the nurse joy's audino since i know someone's bound to bring it up) still showed they could still be strong even without going mega, with AG that was not the case.
Even Infernape was able to bash things into paste without going into it's psycho kaio-ken blaze.
If Greninja comes back, it needs to fight more without relying on that form, to prove that it can be strong on it's natural merits, and thus allow it's transformation to become an extension to be used if necessary, not a crutch it needs in order to fight at all.