Ok just to clarify; what i'm saying is a Pokemon on the main cast probably won't 'die' per say (since Greninja is Ash's) since a good Pokemon dying (not to mention the arguably most popular Pokemon as of now) would hurt people harder than say Lysandre's since they were bad imo. But if it does happen...don't know what people would think ^^;
Thinking about it; if Greninja was only sacrificing the form then surely he'd stay with Ash still right as a normal Greninja, so why would they have to say goodbye? Something definately happens here...if it is the aforementioned thing...that'd be a shock tbh. Unless he stays as Ash-Greninja permanently and leaves, but if he's leaving then how will he stay as Ash-Greninja without Ash?
My question is exactly the same as yours.