According to the ALPA 117 document question 24 a crew can be extended if the PIC and company agree the crew is "fit and the flight can be safely operated if extended". If a fit crew who could operate the flight safely refuses an extension because they don't want to do it they would be opening themselves up to a possible claim of insubordination.

I believe the intent of the rule is to force operators to schedule shorter days (than the old rules) but still allow for schedule interruptions. Instead of saying companies are allowed to schedule x hours and fly y hours they (wisely) included language that forces the crew to assess their fitness for duty and formally affirm it before continuing. It's functionally no different than a fatigue call.