I just think the God Emperor kinda ended it. Nobody would ever go to the extremes Leto II did, and even if they tried, precognition would no longer be a thing. Humanity spreading out into the universe means there wouldn’t ever be a single emperor ruling over all of humanity ever again and thinking machines would never be able to kill all of humanity.

So all of the big things that made Dune what it is were all wrapped up in God Emperor. So to me, it was the ending of the Dune story.