RileyXY1
Young Battle Trainer
They might have to because eventually Ash's list of achievements will grow so massive that he would no longer to justifiably make "newbie mistakes" without full on rebooting the entire continuity of the anime. And at that point, Ash would be no different from a new protagonist because this wouldn't be the same Ash that we've seen for nearly two and a half decades.Yeah, It feels like Ash should have retired after the end of Pokemon Diamond and Pearl, Then Ash would be replaced with a Generic Trainer is known as Joey who starts out with an Unova Starter and this Hypothetical "Joey" would go around for like 100 new Episodes until when they get to Unova Pokemon league.
But no, they had to keep Ash in Pokemon Anime because of marketing and is an iconic figure too.