Actually, they just say that there have been reports of him showing up in cities, but only by moonlight. But that doesn't necessarily mean that he has been written off. Besides, I don't think Game Freaks is going to write off one of the most popular Pokemon so easily...
You're all overanalyzing the word Hero. All it means is that there will be a character that is the main hero of the story. Now, of course Ash will get caught up in this in some way, but the help he offers will be minute in comparison to the grand scheme of what happens in the movie, unless they're throwing in an uber-twist... My point, there's no written guarantee that Ash is the 'Wave Guiding Hero', yet.