Since Ash/Satoshi is pretty much Satoshi Tajiri as a kid in cartoon form, I highly doubt they'll replace him.
Why? Just because he wins a Pokemon League Tournament doesn't mean his story is over. Tournament Champion is not the title he wants, nor is Regional Champion. He wants to be a Pokemon Master. We could very well see much more from Ash.
That battle with Tobias proved that Ash was the second strongest trainer in the tournament. Ash remains the only known person to ever KO Tobias's Darkrai. No one even knew if Tobias had any other Pokemon besides Darkrai.
A new trainer would just be an Ash clone anyway. Ash is the star of the anime and has been since Episode 1.
I'm seeing more and more "replace Ash" and "Mature Pokemon" topics, all because of that trailer for Black 2 and White 2. I think people need to realize that Ash isn't going anywhere, and Pokemon will never go Mature. It's been a kids show from the start. If people can't enjoy it as a kids show, then they should just get out of Pokemon. Pokemon doesn't have to change.