I think you will find it is all about the odds game. At dispatch, if you only have one pack, you plan at a lower level because you are departing under a MEL with reduced redundancy.
Exactly.
We had a similar thing some years back with a 767 operating throughout Asia, we had no pneumatics from one engine and were waiting on spares, operated okay under an MEL with reduced altitude etc until we were to do a Hanoi to Moscow direct flight and we were grounded.
We could NOT operate that flight as there was a large part of the flight around Afghanistan where the minimum safe altitude was something like 14,000 feet (from memory), and had we lost the other engine or its pneumatics we could not have descended low enough (with NO press/aircon) and did not carry enough (pax) oxygen to stay at altitude.