Slavish adherence to the rules is perfect......for people without any ability or sense.
The SOPs are great, most of the time, but this modern culture of writing a rule for everything, and then kicking anyone's ass who doesn't strictly comply is, imho, better suited for ground based activities.
In this case, after the first GA, what was to stop the crew making an independent performance calculation, concluding that their system had gone TU, and then landing?
I'm not suggesting that they just say 'disregard that, we operate to here and we're sure it's fine', then slapping it down.
There should be a place for the careful and diligent crew derived decision.
But there isn't.