I would have thought that JT's post no 10 above makes absolute sense.
The final part of any decision making model GRADE, DODAR etc is to continuously review/evaluate the outcome which includes asking the question ''what if the next one fails?''.
True, an (emergency) descent from FL390 should sort it out but it will probably be ugly; an emergency descent from FL 310 will be less so.
But there are many more factors to consider - but at least we should consider them.
Must go flying
Cheers all,
mcdhu