To me, XY and XY&Z offers the one real last chance to end with a bang for Ash's journey. He's at his best like never before, he has become a leader of the group instead of the focal kid, and has achieved new heights and power that no one has ever done. In my eyes, he has become a Pokémon Master by the end of the Winding Woods episode whether he knows it or not. And because I follow the anime for Ash's adventure, I wasn't too bothered by the lack of continuity references as Ash's independence and newfound progress was enough for me to say he is the same Ash that I follow and look how far he has come in these 20 years. Even if his past companions and Pokémon don't show up to congratulate him, at the very least the audience would finally cheer for the boy of Pallet and we could end his journey on an open ended note (as the road to being the best has no real end).
As for Sun and Moon, it seems to be exhibiting a desperation to regain the young audience. Changing the genre of quests to a genre of school this late in the series is quite shocking to say the least. People say it's too late to change Ash for a new protagonist, but I'm surprised they haven't considered that the same thing could be said for the premise. And now it's changed for better or worse (we have yet to see). But all I can say is that two aspects of Ash's character are gone. He has lost his unique gloves despite having them for over 19 years (even his summer outfit had gloves) and he is no longer traveling to compete in Gyms or anything similar in favor of school. This is something a new character would benefit, not Ash. And there is a phrase to describe this change, though it might be too early to tell.