>>27177283Being the winner of a league would be a pretty impressive step toward becoming a Pokemon Master, but it can't happen to Ash for a few reasons.
While winning a league title wouldn't make him a master, persay, he would be put into the realm of being an elite trainer with some extent of notoriety. The Orange League technically counts, but I feel that because it was "non-canon" that it didn't really impact Ash beyond that arc.
Another issue with him winning a league would mean that his kid appeal would take a nosedive. On the one hand, I'd like it that the anime writers retire the character in favor of a new one entirely, on the other hand it would also give us situations where in the future seasons we'd have Ash still acting like an incompetent boob in battles because the demographic keeps aging out of the show and allowing them to use 15+ year old plot points.