I have to agree with pokemonfan132. Ash has not taken down a single pokemon of any elite four member yet. Would it seem realistic if he just started beating them and the champion assuming he wins this league? Not at all. Perhaps the writers should have Ash battle an elite four, lose but still take down one of the E4'S pokemon, then it might seem realistic if ash can actually beat the elite four after another couple of reigons.
I also agree that many kids who watched DP became Ash fans. and they don't think of Ash as a stale character. How many of us older fans were "bored" of Ash after 2-4 years of watching him? Almost none. There are plenty of kids who have watched Ash for a few years and have become very attached to him, they are not "bored" by him because they haven't seen him for over 10 years. There is a huge difference between replacing Brock or Dawn compared to replacing Ash. No matter how much people insist, Ash has always been the main star of the show.