Brock was with Ash for a whopping 13 seasons (according to pokemon.com) but how long will Dento stay with ash? will he switch every new region like the female protaginists or will he stay for a long time.
Personally i hope he leaves and brock comes back :P
Iris has more of a chance staying longer than Dent. She has a fresher personality that the writers can't afford to lose. If she is replaced, it'll be a clone of her or May/Dawn, and who would want that? Not that May/Dawn were bad, I liked them, but having that personality type after Iris' is never good. So I think Iris will stick around for a while, Dent, not so much. He'll probably get bored.