I know it may be annoying that Ash keeps losing on every region he enter the tournament (save for Orange League, in which he won), the Kalos league that he made it the final was one of the nearest potential, even using Greninja (along with its bond phenomenon) could have helped him winning for sure, but it wasn't as it used to be. The Alola League may be the one that Ash might lose too, since anime boosts popularity both Japan and internationally, we won't seeing him achieve his dream. We need to wait Gen 10, which could be the last Generation to be made, this kind of final generation could help Ash winning the league and even achieve his goal of being the Pokemon Master. The Pokemon Company never revealed the details about what kind Pokemon Master is, but Masamitsu Hidaka stated in an interview that if Ash had won the league, the series could end. But despite all of this, we need to keep waiting until Gen 10, the final Gen Pokemon, comes and Ash finally achieves his goals.
Gen 10 could be the only remaining for him to win, that's my theory.
(sorry for my poor English)