A thought....
The oxygen system is there in case of an emergency.... right?
Bird strike up front going through the window or the front bulkhead.
A window blowing out through a fault or a pax going crazy.
A fault in a door, like a cargo door blowing out.
A structural failure (with the 737 cabriolet as an extreme).
So the oxygen system is there as a backup/emergency system for such rather rare occurrences.
In other words, we thought we had two low-probability events: a rapid decompression AND an oxygen system failure, before we had a real problem
But maybe nobody did the sums for the case where the oxygen system itself was going to be at the origin of the decompression..... ?