In that same interview, James mentioned that he wanted Sidney’s departure to be a gruesome one involving murder and a new vicar arriving to solve the mystery. Instead, the character introduces the new Reverend Will Davenport and the two solve one final mystery together.

“It’s been a privilege to play such a wonderful character, but I feel like there are other vicars, other conflicted souls to explore,” he said at the time. “So it’s time to introduce Tom Brittney as Reverend Will Davenport.”

Source: https://www.distractify.com/p/why-did-sidney-chambers-leave-grantchester