Um, nope, if you're trying to imply that it's service is forever needed in Kalos then know that you're wrong, it was never even once stated that Greninja was released to be the protector of Kalos, Greninja was released to deal with the negative energy vines that remained in Kalos after the Team Flare incident, because Zygarde asked for Greninja's help in that matter. Greninja has a special ability to sense the negative energy vines, which Zygarde cannot do. So it's for that reason that Greninja was released to destroy the negative energy vines along with Zygarde. After Greninja is finished dealing with the vines, it literally has no reason to be in Kalos anymore and then it would make perfect sense for it to come back at Oak's lab. It's Zygarde's role to be the protector of Kalos as a whole, not Greninja's. Remember that it's confirmed that Oak currently has Greninja's Pokemon, so the door's always open for Greninja to come back there. Greninja can just go back to Sycamore (whom it knows very well, as a Froakie it originally stayed at Sycamore's lab) and his lab from where Sycamore can easily transfer Greninja back to Oak's lab via the Pokeball trasport system.
If the writers do want to bring Greninja back, they will, period. And I believe that's more likely than not, because Greninja is one of the most fan-favourite, popular and iconic Pokemon of Ash, one of his regional aces. The power of popularity and fanservice should never be underestimated, it's the same reason why we got to see Charizard in BW again.