After seeing some things in the anime, I've changed my mind; perhaps Ash
will win the Sinnoh League.
Supporting arguments:
-When Lucian was introduced in "Elite Meet & Greet!", he mentioned something that has never been mentioned before (but we've seen in the
very first episode):
The Champion League. He went into detail, saying that "Only those Trainers who have won in the respective region's league can participate in the Champion League, and have to defeat the Elite Four, then the Champion, to become the Champion." That brings up the question: Why is this explanation necessary? XD
The entire argument of "Why is this necessary?" can bring up much discussion on the answer;
Ash will win the Sinnoh League, and we'll see him compete in (and lose) the Champion League (but somehow, he'll fight all four of the Elite Four on the way, for the sake of tangency with the video games.)
Frankly, I'd love to see that. :)