I agree it's a natural extension of its power and I don't have any problem with It using him. I just hate it immediately had it as a greninja. I wish we got some time with it evolved as a greninja and doing things on its own and defeating mons, and then Ash working together with greninja to unlock that power and then use it. I wished there was an intermediate between frogadier evolving and Ash greninja activating. We didn't get base greninja to do anything. It's not an argument against its strength, AG and Grenjnja's feat at the end of the day are same, it's just how wasted Base Greninja was. I really wish we got to see it for some time as Base Greninja before then unlocking this form, and I wish we had a proper backstory and explanation of this transformation considering how Integral it was to the story, and a proper climax moment for him, like defeating Charizard (Sceptile didn't do it for me).
Not denying Grenjnja's strength. To counter your frogadier point
It's the same as Torracat. We know its strong, and so when it evolved to Incineroar we know it's still strong too. But we WANT to see it battle! And show feats of its own. Same with base greninja. I wish we saw greninja doing things on its own as well before unlocking Ash greninja. I have no problem on it relying on that form to defeat its main opponents like Infernape with Blaze, but LIKE Infernape I wish we saw it do stuff with base greninja as well before transforming into its form. We didn't get enough of that. Overall Infernapes arc was much better handled
The bolded sentences pretty much are what i'm getting at when it comes to AG, and why I don't like it.
Greninja in it's base form never got to prove itself like others did.
Especially when you consider that this was the first fully evolved water starter Ash ever had, I wanted it to be handled right but the transformation overtook and destroy it.
Yes Infernape used Blaze when needed, and Pikachu Z-moves, but they still showed they could destroy stuff without them, and used those attributes when it was necessary, I wanted to see Greninja do that, but it didn't get the chance as it transformed nearly everytime we saw it.
that is why i consider it's power a crutch and not an extension, does it take away from it's feats, no. But your always left wondering of how strong it is without transforming, because it wasn't able to prove it's strength on a natural level.