They said, "If you leave, it will be cancelled."
Consequently, he stayed longer than he planned and the show remained on the air.
He finally decided to leave and they stopped making it. As far as is known, he didn't leave because it was cancelled. It appears to be the other way around as best as can be determined. They stopped making it because he left and there was no one on hand to produce it. (So, yes, that's exactly what I meant and you read it correctly the first time.)
There are some BBC people on record indicating that it wasn't their intention to permanently cancel the show. Sorry, I'm forgetting who at the moment. I guess they thought they'd find someone at some point. Verity Lambert (no longer at the BBC) was interested around then. But I guess her efforts got shot down because Worldwide wanted to make a movie. But it does get complicated!
