I agree with everyone, as I doubt they would really replace him at this point - they had plenty of chances to do so if they wanted in the past. It's a little hard for me to put into words (silly, I know), but I also feel that, now that he's been their star for 6 generations of games, it's a bit too late.
But I do think it wouldn't be a bad idea. It's not that I don't like Ash - I just feel like we've seen enough of him. He's matured a lot since the beginning of the series (despite somehow magically still being ten), and I think it's especially visible in the last few seasons. In the very first season, Ash was kinda bad at everything, and often made silly mistakes, but now he's well-respected by most people he comes by and is very skilled with dealing with Pokemon - why not switch to someone else and let them go trough the same journey, in their unique own way, rather then beating the dead horse by showing us how brave and devoted Ash is now?
I remember when we first got introduced to May - we saw her driving to get her Pokemon and her commenting how she'd really rather just travel... now that I think back on it, I wish May became the main protagonist at that point over Ash. I believe that it would be an interesting new dynamic - while Ash was very excited at the thought of becoming the Pokemon master at the start but was still immature and kinda dumb - May would have different issues about herself to tackle on her way.
Best we can perhaps hope for is a spin-off, tho.
