Consider the scenario with 1 pack available.
To operate this pack at FL250 where the ambient air is considerably 'warmer' than FL400, this pack would be working hard.
If you operated at FL400 with 1 pack, the same pack would not be working as hard, thus less prone to falling over.
Regulatory authorities place the restriction of FL250 for depressurisation considerations (I guess).
I would certainly continue to a higher than FL250 altitude in a single pack operation.