BlunderBus
And that is the exact reason why a captain can decline an extension to a FDP. It is there in black and white. I personally have flown with a captain that declined a request to extend a FDP on a ULH opp for a similar reason. No one questioned his decision and there were no ramifications later on. His decision as far as I was concerned was based on valid reasoning.
24.4 After receiving a request, the aircraft Commander, taking into consideration all relevant factors, including the circumstances of the other crew members, including Cabin Crew members, and the over-riding consideration of safety, will inform the Company of his/her decision. The aircraft Commander may decline the request and also may elect to work less than the full extent of the provisions of 24.5, 24.6 or 24.7. The Commander’s decision in such matters will be final and unquestioned.