To be honest, after the move they pulled in choosing Iris over the new female protagonist, I have a feeling Dento and Iris will stick around just as long or even longer than Brock and Misty did in the originals. If it's true that Ash has indeed moved to Isshu with his mom, then if he ever has to return home, it's within the same region as Iris and Dento now; like it was with Brock and Misty.
Maybe Iris might stick around like Misty did, and eventually return home to take her place as the 8th Gym Leader in Isshu. However, I believe Dento will be around just as long as Brock. I mean, what's the point of finding a new replacement to Brock's role every new generation?