Same. They hyped up that final battle so much, especially that giant orange water shuriken thingy. Also Greninja is my favourite Pokemon, so that makes me even more salty (HOW DID A GRENINJA NOT DEFEAT A CHARIZARD?!??)
But yeah I agree, if Ash just leaves Greninja behind that would be so dumb. I mean, they worked really hard to finally be able to go synchro and make use of their full power, but then all that would go to waste! But if Ash did go to Alola, I don't think he would take Greninja. So maybe there will be a new cast of protagonists?