I agree. But if Greninja returned, I would like to see a scenario, or even arc, where Ash has to learn to use more strategy with him and actually see that even as Ash-Greninja, he isn't invincible and can be beaten, even by a non-Mega Evolved, non-Dynamax/Gigantamaxed Pokémon. I'll use an example from something I wrote a while back. Let's say that Ash-Greninja is facing a Grass-type Pokémon. I'd have it where the Grass Trainer has the Grass-type use Sleep Powder to cover the field. Since Ash-Greninja is a close to mid-ranged fighter, he's screwed because to attack, he'd have to go into the cloud of Sleep Powder, which would put him to sleep and allow the Grass-type to wail on him with super-effective attacks. and if he uses Double Team to go evasive, prompting the use of Sleep Powder to protect the Grass-type Pokémon, and then uses the Water Shuriken, even the Orange Water Shuriken, well, its effects on the cloud now give away Ash-Greninja's location, meaning that if the Grass-type has a ranged Grass-type attack, it has a good chance of scoring a direct hit on Ash-Greninja and taking it down, or significantly weakening it and neutralizing Double Team.
It could teach Ash some valuable lessons that would not only help Greninja become that much more powerful and dangerous, but can also be used for his other Pokémon, particularly the ones that job a little too much for the fandom's taste.