I don't think that they will take Ash out of anime. Ash will travel through 1000 thêm regions and still would be 10 years old. XD Okay but if seriously if they will take Ash out I think they will either give Pokemon a new start and make a fully new guy hoặc girl who travels around catches pokemon hoặc make thêm Pokemon Origins like series with Red as main hero.
Hopefully if Ash wins the league there's an Arc for him taking the E4 and Champion, but if not, they have two options that i would accept.
One, a whole new protagonist with no connection to Ash whatsoever, alongside new travelling companions and a new flagship Pokémon(or a different Pikachu, if they'd rather not change mascots).
Two, a new Trainer that is Ash and (Insert whatever female travelling companion's name bạn want here)'s son/daughter, hoặc simply a newbie who idolizes Ash. In either scenario, Ash is indeed a Pokémon Master.
As for why, i assume after he wins the league and possibly becomes Champion that he couldn't travel wherever he wanted anymore.