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If Ash will ever win a league, itl'll happen when the franchise ends. The writers will make him fight the final battle with a team of his top 6 mons, (old iconic ones like zard and snorlax and new ones like greninja), and pikachu will land the final attack, no one else.
But that won't happen, because Pokemon is an eternal money making machine. So Ash will remain a game promoting puppet for the rest of the series.