I just want to go ahead and get this out there: Pokemon has been around for 12 years. Do you realize how long that is? When the series started, I was in 4th grade. I was maybe...9 years old. I am 21 now.
Anyway, I've got some knowledge bombs to drop, and I hope this doesn't blow anyone's mind or piss anyone off too badly, but let's go ahead and have some things said.
#1. Ash will not "leave" when Sinnoh ends.
Firstly, he was already the champion of Orange League and Kanto Battle Frontier, and was offered the chance to become a Frontier Brain and declined it. If they wanted to end his journey, they would have done so back then.
Secondly, If they did end his journey, they would basically be committing suicide in the anime department. I think from a business perspective, it would be better to keep him around and continue to milk him. He is incredibly popular, and a familiar face. I am much more likely to buy my kid something involving Ash than I am to buy him something involving someone else.
Basically, he has been around for so long, I want my kid to get into Pokemon, if for no other reason than to share the joy that I felt as a child. However, if I can't sit down and be familiar with at least one of the main characters, then that kills the whole "bonding" aspect of Pokemon.
#2. A time jump is incredibly unlikely.
How many years has Pokemon been around? How many years has Ash been around? How frequent are the League Tournaments? They seem like a yearly event to me. Kinda like the Superbowl or something of that sort.
He has collected 42 badges so far, and will be getting the 43rd badge very soon. He has completed the Kanto Frontier, competed in 4 leagues, and will be competing in his 5th league very shortly. This is not including the 23 other events which he participated in. With all of this that he has done, there is almost no reason for them to have a time jump in my opinion.
A more likely possibility is them simply saying that he is 16 now and maybe even, "Go see your father. He is in Isshu." That would be something new, interesting, and while he is there, he can battle gym leaders or something. Maybe he could "follow his father's footsteps" or something, or work his way up because he is trying to win the chance to have a battle with his dad?
#3. The anime is based upon the games.
If the games have 16 year old main characters, then I could easily see them making Ash 16. "Oh, but he still looks so young!" You know how they change his clothes? Well, change his clothes and redo his model to make him look older while you're at it. Nothing wrong with that.
Both Gary and Ash already look older than they did when they started. Proof?
The difference isn't dramatic, but they do look older and more mature.
New main character seems ridiculous.
Time jump would be super weird, all things considered.
The anime is based upon the games, so expect them to line up (generally).
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I will not deny that I could
be wrong, but I truly believe that they will not replace him. It's just bad business. I can't think of a single series that has done a successful reboot of a series. I would hate to see Nintendo/Pokemon shoot themselves in the foot.