Ash, to me, is something of a harsh life lesson for kids, that no matter how good you think you are, there is always going to be someone better than you...as evidenced by the fact that he's never won a league competition, despite frequently having a very strong lineup (Sinnoh was the most painful example of this, where it took a DARKRAI and then a LATIOS to bring him down, as opposed to non-Legendary Pokemon as had been the case every other time) and the entire anime building up to it. Replacing him, at least before he's actually won something of significance, would leave his story incomplete, at least in my opinion. If Ash were to be replaced, I'd want to see him win for a change; heaven knows he deserves it after so many failures it's unbelievable. It's a good thing his mind is effectively wiped with each new season, because if it weren't he'd have some serious confidence issues by now.
I have to admit, I'd like to see a fresh face taking the spotlight instead of Ash. But I remember being extremely upset as a child when Brock was replaced by Tracey, and something tells me a lot of the younger fans would be equally as upset...and it'd be really jarring. Ash is the main character of the franchise; it's built up around him. Now, a spinoff series with new characters might be a good idea (like The Legend of Thunder episodes/movie) but replace him entirely? Gotta say no to this one.