End his story in 2026, the 30th Anniversary of Pokemon.
Assuming, Sword and Shield lasts 3 years. 2020 to 2022. Drag the next Generation for 4 years; they have done this before...
Rush through the regular story in 2.5 years main league. Then have a timeskip of like 2 to 5 years; it doesn't matter, he is 10 years old... then return him to Kanto to conclude his story.
He can show off his beefed up Pokemon he trained during the timeskip. For instance, surprise us by sending out a Venusaur, Blastoise, and Garchomp. Surprise us with Pidgeot and Primeape.
Have him beat officially Team Rocket, team up with Mewtwo for some epic conclusion, then have him return to the Kanto League, beat the Elite 4, and make him the Champion of Kanto.
Then reset the Pokemon Anime in the next season, with a new character, and I guess Pikachu... but no more Ash, Ash is now only known as a legend that saved the Pokemon world.
2026 is the 30th Anniversary of Pokemon. Assuming kids that grew up in Gen 1 was around 10 years old in 1996. They be 40 years old already. It should be a last push for kanto pandering and the people that grew up with Ash.
The problem with Ash is that he is multi generational mascot character. You can't make him strong, you can't make him weak him, you can't progress him without deprogressing him. The Anime has to go on, but it's hard to write Ash as a character that develops if there is no end to his development. He can't be too mature because young kids are watching him. He can't be too dumb because 30 year old adults are watching him. The writers essentially wrote themselves to a corner, and it's worst you can't replace him because he has become too iconic...