I WISH that they would get rid of Ash, but he is the main protagonist so don't bet on it. It would take like a huge drop in popularity that just kept going for something like that to happen. Like they'd drop the other two characters faster than they would Ash, then wait like another x amount of years before dropping Ash if something like that ever happened..and I doubt it will.
I wonder if they will keep Dawn or Brock when Gen.5 starts. They've been in Sinnoh for a really long time and Dawn still seems like a preemie to me (and I'd miss her piplup) so she can't go. Brock can't go either because he's the only good character left! Trust me, they will more than likely drop one of these two.