I highly doubt that Ash will ever leave the anime, since it's basically his journey that the anime is all about. While I know a lot of people are getting tired of Ash because he's arguably pretty stupid most of the time, keeps forgetting type advantages, Pokemon he's already met, etc., and still manages to lose to newbie trainers despite reaching high ranks in various tournaments and such. But fact just is, the show is all about Ash. Giving the show a new protagonist would be like giving the anime Naruto a new protagonist, or Bleach, or whatever. The show may not be titled "Ash and his Pokemon", but it's still all about him.
And comeon, while a big chunk of the older audience would like to see Ash gone, he still appeals to the younger viewers, and they aren't really as judgmental, and don't seem to see continuous and repetitive mistakes as anything bad, and since the target audience of the show is the younger viewers, no way are they gonna risk it all by giving the show a new and more mature main character. If the demand is for a "cooler" show for the older audience, kinda like the B2W2 promo to appeal to more people, then I think it's too late. The whole franchise is already regarded by many as childish, so a reboot of the anime would most likely not help bring new fans to the franchise, unfortunately.
So yeah, don't count on Ash leaving anytime soon.