Ash has never released a starter pokemon.
Humm, hello?!! Have you forgotten Charizard? Even although he returned, Ash still released it.
Anyway. Guys, I have a horrible feeling about this.
First of all, I can't believe the writers are making Ash leave one of his greatest Pokemon behind (probably even surpassing Charizard). However, I want to believe there's always a chance for him to return. I mean, if Greninja goes to Professor Sycamore's lab, and if there's connection between Kalos and Kanto (which could be because of Ash's comment in the Kalos League about requesting some of his Pokemon to Professor Oak), I guess it could be possible to Greninja to return someday in the future. It would be like Charizard, after all. However, I want to believe that, even if Greninja leaves Ash's team, he could return in the next season, at Alola, somehow.
But anyway, this new has made me suspect something really horrible and I truly hope to be wrong. What if... ALL OF ASH'S POKEMON FROM KALOS LEAVE?!!! Now this would make me want to smash the stupid head of the writers, but think about it:
+ Greninja: Apparently, he's staying in Kalos in case Xerosic or any other member of the Team Flare tries to do something again.
+ Goodra: He could return to his homeland, the Wetlands.
+ Noivern: They could find Noivern's natural family, explaining what in blazes his egg was doing all alone in the forest, when Ash's group found it.
+ Hawlucha: He could return to his natural forest as its Champion, even when Machamp already took care of it when he decided to leave with Ash.
+ Talonflame: This is the only one who wouldn't have a reason to leave.
Bad enough is that Ash is losing Greninja, but his whole Kalos' team?!!! Then the whole journey of Pokemon X-Y-Z would have been for nothing!!!