At what point does The Office (US version) become unwatchable?

I don't know. I find Sun + Moon (especially Rotom Dex) very entertaining, but I didn't care for much of the rest.

I stopped watching the first time they went to a new region and Ash decided to get rid of all his pokemon except Pikachu. I was upset every time Ash left a good pokemon behind (Butterfree is the one that stands out in my memory. Maybe Charizard too?), so leaving his whole party behind with Prof. Oak ... meh.

I understand from a storytelling perspective that the writers can't continue with Ash's final team forever. I remember hoping that they would solve that by instead writing Ash out of the show and transition to entirely different characters for each new region. That would have been a rough change the first time, but would be better in the long run. Solves the bizarre continuity issue of Ash never aging, and could still leave the window open for cameos from returning older versions of characters.