Replacement only alleviates the symptom of the problems and not the problems themselves.
For Ash to leave, you need to give finality to him while justifying the existence of the next protagonist.
How do you justify the existence of a new protagonist? Well, for starters, making an Ash clone is not the way to go which I totally foresee.
When you see other shows that change cast every year like the various Tokusatsu shows, you notice a pattern. Despite introducing the same formula and tropes, they always build a new mythology with new arcs for their characters. No one who has seen Samurai Sentai Shinkenger and Tensou Sentai Goseiger will say they are the exact same shows with the exact same characters. With Pokemon, you run into some common protagonist replacement problems.
The first option is to make this a protagonist to a sequel. You can easily run into the Davis Problem where your protagonist is so similar to the old one and such a stand-in for the old one that the protagonist comes off as dull and uninteresting. The first time you tell your story, it's a tragedy. The second time you try to pull this on the audience, it's a farce.
The second option is to make an isolated mythology. You leave it ambiguous or explicit that there is no connection between old and new characters and mythologies. This is the route Tokusatsu takes nowadays. The issue with this is that you risk diminishing popularity per series. There's a reason why Digimon fell victim to this. If a series is really bad, you can kill a franchise. I'm looking at you Digimon Frontier and Lupinranger vs. Pataranger. Each series would be courting cancellation. Before anyone can say "this is Pokemon and won't get canned," Super Sentai has 45 series and skirted cancellation multiple times.
