Honestly, although several people working on the anime wanted Ash to leave many years earlier, looking only at the final results, I would almost believe someone if they said that this had been planned the whole time. Our boy kept climbing the mountain, sometimes unable to advance, occasionally backsliding to where he was before, but he kept climbing further and further until reaching the peak. He keeps getting better takes a step back to then push on to be the very best. Makes Unova feel vital to the story.