The Kanto episodes was likely when they forumlated the idea of Ash winning the Pokémon League if they didn't plan it from the beginning. Think about it, before the Kanto episodes, there was no indication that a Pokémon League would take place. The main goal was, after all, to graduate from Pokémon School. They could have avoided the League altogether and thus the question of Ash winning the League. Then the Kanto episodes happened and we heard about Kukui's plan to bring the Pokémon League to Alola. Suddenly, questions regarding Ash's position in the Pokémon League would inevitably be brought up and we know the last League had Ash at the Finals.
There was no going back after that. It was either victory or finals again. He could not regress back to Top 4 or Top 8 again as it would stain SM's reputation. And since everyone expresses interest to partake in the League despite the fact that many don't even have 2, let alone 3 or 6, Pokémon at hand, the competition would be much easier for Ash and the animators.