One problem with the definitions used in discretion extension decisions is the use of the wording. "Are all crew members fit to continue the duty beyond the FTL's?"
In other spheres the use of fatigue to call un-fit for duty is the threshold. Too tired is not enough. In the discretion extension case fatigue is not mentioned; it is fit to....
Management have the opinion that if you are not fatigued then you are fit, and you do not transition from fit to fatigued at the FTL limit. You need to make the decision, perhaps 2-3 hours in advance, how you will feel at the end of the extension. Impossible. You can only say how you feel now, and speculate. Managers don't believe you can be tired enough at end of FTL to decide accurately and so should continue as required.
What they don't have any knowledge of, nor interest in, is what duties you have worked previously, how much sleep you achieved before this one, what is your overall condition. They take it as a given that their non-fatiguing roster patterns, and your immaculate sleep management family programs, delivered you to work 100% fit & sharp. The extra mile should be taken in your stride and any refusal is tantamount to a silent strike.
That attitude needs to be addressed and changed PDQ and it will best happen from the XAA's or even EASA. Dream on.