Illusion on 19 July 2016
Ash actually won the Orange Island League back in Season 2. While this was not technically a league in the tournament sense it still made him a champion for a short time.
I don't think that there would be any issues for the anime if he won a full-fledged Pokemon league. He would just get beat by some rival trainer immediately after the awards who is using a new Pokemon from Gen 7 and then that would force the series to go on.
